Buying swimwear by the container is a bet on consistency. The print looks right on the salesman sample, the hand feel is good, the price works, and then the first batch comes back from a hotel pool program with bleached panels and a white lining that turned pink. That is a colorfastness failure, and in bulk it is expensive. This swimwear colorfastness testing guide sets out which tests actually matter — chlorine, light, wash, and crocking — and what a passing result looks like on a real report.
This guide is written from the factory floor. We run fastness tests on every dye lot before it cuts, and we have rebuilt more than one buyer’s spec sheet after a bad season. Below you will find what each fastness test measures, the grades that actually protect your margin, the ISO methods behind them, and the exact lines to put in your tech pack and purchase order. If you plan wholesale programs, our B2B wholesale tips cover the wider buying picture.
What Colorfastness Means (and Why It Sinks Swimwear ROI)
Colorfastness is a fabric’s resistance to two things: losing its own shade (fading) and transferring dye to whatever touches it (bleeding or staining). A swimsuit is judged on both. A navy shell that fades to washed-out grey still sells poorly; a red shell that stains a white lining pink is a defect a customer returns on sight.
For swimwear the stress is harder than for normal apparel. The garment sits in chlorinated pool water, salt water, and strong sun, then gets hand-washed, rolled in a beach bag, and rubbed against skin for hours. Each of those conditions pulls dye out of the fiber or moves it sideways into a neighboring panel.
The return math is what hurts a B2B buyer. A single defective dye lot can mean thousands of units with the same flaw. You eat the returns, the restocking, the chargebacks, and the lost reorder from a retail account that lost confidence. Seasonal prints cannot be re-run cheaply once the window closes. Specifying fastness up front costs almost nothing compared with a recall.
Fabric choice drives the ceiling on fastness, which is why we always tie this conversation back to our swimwear fabric selection guide. A nylon ground dyed with the right chemistry will hold chlorine far better than a cotton blend, and recycled nylon behaves like its virgin counterpart when the same dye system is used.
Chlorine Fastness — The Pool Killer
Chlorine is an oxidant. It attacks the bonds that hold dye inside the fiber, and swim training lines, resort wear, and kids’ programs live in it daily. Two ISO methods cover this: ISO 105-E03 measures colour fastness to chlorinated water, and ISO 105-E06 measures fastness to chlorinated pool water using active chlorine at controlled concentrations.
We test at two levels because “a pool” means different things. A well-maintained facility runs around 20 mg/L of available chlorine. A busy public or hotel pool that is dosed hard can sit near 50 mg/L, and some spike higher. A suit that passes at 20 mg/L can still fail at 50 mg/L.
For training and competition swimwear we want Grade 4 at 50 mg/L. Fashion and resort lines that rarely see a pool can live with Grade 4 at 20 mg/L. The dye system decides the result: disperse dyes on polyester resist chlorine well, but nylon is the workhorse of swimwear, so we specify metal-complex acid dyes with an after-treatment that locks the color. Recycled nylon and GRS-certified recycled polyamide take the same chemistry and reach the same grades.
If your program is built for pools, start from our chlorine-resistant swimwear wholesale guide so the construction and the dye spec are matched to the end use.
| Test | Standard | Chlorine level | Min grade for swimwear | Typical use |
|---|---|---|---|---|
| Chlorinated water | ISO 105-E03 | 20 mg/L | 4 | Fashion, resort |
| Chlorinated pool water | ISO 105-E06 | 50 mg/L | 4 | Training, kids |
| Chlorinated pool water | ISO 105-E06 | 100 mg/L | 3–4 | Harsh public pools |
Light / Fade Fastness (UV Exposure)
Sunlight breaks the chromophores inside a dye molecule, and a suit left to dry on a balcony or worn on a beach holiday takes a steady dose of UV. Light fastness is measured by ISO 105-B02 under a xenon-arc lamp and graded against Blue Wool references on a 1–8 scale. Swimwear should hold Grade 4 at minimum; bright fashion colors and anything marketed as fade-resistant should reach Grade 5–6.
The test runs in a controlled chamber with set temperature and humidity, exposing the sample to a defined amount of xenon-arc light equivalent to a known outdoor dose. We typically run to a level that corresponds to Blue Wool 4–6 depending on the line. Pastel shades read as fading faster at the same grade, so we set the bar higher for light colors.
Print method matters here. Digital sublimation and heat-transfer prints sit as pigment near the surface and generally resist UV well, while a dyed ground depends entirely on the dye class. A UV-absorbing finish can help, but it is a supplement, not a substitute, for a dye that is inherently light-stable.
Wash Fastness
Swimwear is labeled hand-wash for good reason, but customers still put it in a machine, and a dye that bleeds on the first wash is a defect no matter what the care label says. Wash fastness follows ISO 105-C06, which simulates domestic and commercial laundering. We run a single wash at 40°C with the specified detergent and steel balls, then grade both the change in the sample and the staining on adjacent multifiber cloth (cotton, nylon, and polyester).
For swimwear we want Grade 4 or better on both change and staining. This test also catches printed goods: a sublimation print that bleeds into the wash water, or a screen print whose binder releases pigment, shows up here. If a style is color-blocked — a bright shell against a white lining — wash fastness is where you find out whether the two will stay separate through a season of care.
Crocking (Rub) Fastness
Crocking is color loss from rubbing, measured by ISO 105-X12 with a standard crockmeter that rubs a white test cloth against the fabric, dry and wet. Dry crocking is the easy one; wet crocking is harder because water mobilizes loose dye and carries it onto the cloth. For swimwear we target Grade 4–5 dry and Grade 3–4 wet, with training and kids’ lines held closer to Grade 4 wet.
This is the test that catches transfer you feel, not just see. A white lining that picks up shell color where the two panels touch, a print edge that rubs off on skin, or a dark-on-light color block that bleeds at the seam are all crocking failures. Wet crocking is the one we watch most, because a damp suit in a beach bag rubs against itself for hours.
Lining choice is part of the fix. A tight, well-fixed lining dyed to the same fastness spec as the shell will not pick up color; a cheap lining with loose dye will. We treat crocking as a construction spec, not just a dye spec.
How Fastness Is Graded (Grey Scale 1–5)
Every fastness result is graded on a grey scale from 1 to 5. ISO 105-A02 governs the scale for change in colour; ISO 105-A03 governs the scale for staining onto an adjacent fabric. Five means no visible change; one means severe change or heavy staining. Half grades — 4–5, 3–4 — are used in practice because real fabrics rarely land exactly on a whole number.
A 5 is a perfect result and is common for a well-controlled dye lot. A 4 is a slight, retail-acceptable change for fashion lines. A 3 is noticeable and borderline: fine for a one-wear resort piece, not for training gear. Anything at 2 or 1 is a reject. The grade is a judgment made by eye under standardized D65 daylight against the physical reference scale, which is why consistent lab lighting matters as much as the test itself.
| Grade | Change in colour | What it means for your line |
|---|---|---|
| 5 | None | Perfect, retail-ready |
| 4–5 | Negligible | Safe for any line |
| 4 | Slight | Acceptable for fashion |
| 3 | Noticeable | Borderline; reject for training |
| 2 | Considerable | Fail |
| 1 | Severe | Fail |
Specifying Fastness in Your Tech Pack and PO
The single biggest mistake we see is the word “colorfast” written alone in a spec. It means nothing. A real spec names the test, the standard, the condition, and the minimum grade. “Chlorine fastness, ISO 105-E06, Grade 4 at 50 mg/L” is a spec a dye house can hit and a factory can verify. “Good color” is a complaint waiting to happen.
Put the fastness table in the tech pack next to the construction notes, and repeat the key lines in the purchase order so the mill, the dye house, and the QC team are graded against the same numbers. Our custom swimwear tech pack essentials walk through the full document; fastness is one block you should never leave blank.
Tie the spec to the product type. Training gear needs the hard chlorine and crocking numbers; a resort cover-up needs less. Care and fiber labels also fall under this part of the build, and EU buyers must meet EU 1007/2011 and REACH swimwear labeling rules for fiber content and restricted substances, including azo dyes. Specifying OEKO-TEX certified and GRS-certified recycled polyamide inputs keeps you on the right side of those rules while you lock in fastness.
| Product type | Chlorine (E06) | Light (B02) | Wash (C06) | Crock (X12 dry / wet) |
|---|---|---|---|---|
| Training / competition | ≥4 @ 50 mg/L | ≥5 | ≥4 | 4–5 / 4 |
| Resort / fashion | ≥4 @ 20 mg/L | ≥4 | ≥4 | 4–5 / 3–4 |
| Kids’ swim | ≥4 @ 50 mg/L | ≥4–5 | ≥4 | 4–5 / 4 |
| Rashguard (UV) | ≥4 @ 20 mg/L | ≥6 | ≥4 | 4–5 / 4 |
How a Factory Tests Fastness
The process starts with sample preparation. We cut swatches from the actual production dye lot — not a separate lab sample — and condition them at 65% relative humidity and 20°C so the fibers hold a stable moisture level. Each ISO method then runs on its own: chlorine immersion for E03 and E06, xenon-arc exposure for B02, a laundered cycle for C06, and crockmeter rubbing for X12.
Assessment is done by eye under D65 daylight against the grey scale references, recorded per sample with the grade and a photo. In-house testing catches lot-to-lot drift before cutting; for retail or private-label programs that need proof, we send confirmation sets to a third-party lab such as SGS, Intertek, or Bureau Veritas and attach the report to the shipment file.
Where this meets your QC plan is direct: fastness is one input to the same inspection routine that grades the finished goods. Our swimwear QC inspection AQL guide explains how we set sample sizes and accept or reject a lot, and fastness results feed that call. A suit can pass measurement and workmanship and still fail the season on color, so we treat fastness as a separate, non-negotiable gate.
Frequently asked questions
What chlorine concentration should my swimwear be tested at?
Test at both 20 mg/L and 50 mg/L under ISO 105-E06. Use 20 mg/L for fashion and resort lines and 50 mg/L for training, kids, and any program that will see public pools. Specifying only one level hides failures that show up in real use.
Is Grade 4 enough for training swimwear?
Grade 4 at 50 mg/L is the practical floor for training and competition suits. Below that, color loss becomes visible within a season of regular pool use. Performance lines aimed at daily training should hold Grade 4 wet crocking as well.
Do recycled nylon and recycled polyamide pass fastness as well as virgin fibers?
Yes, when the same dye chemistry and after-treatment are used. Recycled nylon and GRS-certified recycled polyamide are the same polymer as virgin material, so fastness is set by the dye system, not by the source of the fiber. Ask for the grade, not the recycled claim.
Why does my white lining turn pink after wear?
That is wet crocking failure. Loose dye from the shell transfers onto the lining where the panels rub, especially when the suit is damp. The fix is a tighter dye fix and a lining dyed to the same fastness spec, verified by ISO 105-X12 wet rubbing.
How many washes should swimwear color survive?
We spec Grade 4 or better on ISO 105-C06 at 40°C, which covers normal hand and gentle machine washing for the life of the garment. A suit that bleeds on the first wash fails the spec regardless of the care label.
Does sublimation printing fade in chlorine?
Sublimation and heat-transfer prints resist UV and washing well because the pigment sits at the surface, but the ground fabric underneath still faces chlorine. Printed swimwear should meet the same chlorine grade as a solid dyed style for the parts that are not printed.
What ISO standards apply to swimness testing?
The core set is ISO 105-E03 and ISO 105-E06 for chlorine, ISO 105-B02 for light, ISO 105-C06 for washing, ISO 105-X12 for crocking, and ISO 105-A02 and A03 for grey scale grading. These are the methods a credible lab will cite on a report. Background on the concept is covered on the Colour fastness reference page.
Can fastness be improved after the fabric is made?
Sometimes, through a re-fix or a clearing wash, but it is unreliable and costly in bulk. The right time to lock fastness is at dye selection and the first lot approval, not after a finished roll is already cut.
How do I write a fastness spec in my tech pack?
Name the test, standard, condition, and minimum grade for each method, and repeat the key lines in the purchase order. Use the product-type table above as a starting point and adjust by end use. Leave no method blank.
Who performs fastness testing, the factory or a third party?
Both. The factory runs in-house lot checks before cutting to catch drift; a third-party lab such as SGS, Intertek, or Bureau Veritas confirms and issues the report your retail or private-label account may require. Keep both records with the shipment file.
Conclusion
Colorfastness is not a finishing touch; it is the difference between a swimwear program that reorders and one that gets returned. The four tests that matter — chlorine, light, wash, and crocking — each map to a real failure you can see in the field, and each is graded on a grey scale you can write into a spec. Put numbers, not adjectives, in your tech pack and purchase order, match the grade to the product type, and treat fastness as a separate gate in your QC plan. Do that, and the season’s color stays where it belongs.
Talk to our team
If you are building a private-label or OEM swimwear program and want fastness written into the spec from the first sample, request a quote and our team will walk you through dye selection, test levels, and lot approval against your target grades.