How Do Colour-Changing Swim Shorts Work?

You jump in wearing one colour and climb out wearing another. It looks like a magic trick, but the idea behind colour-changing swim shorts is surprisingly simple. Once you know how it works, you can have more fun with them and keep the effect going for longer.

The short answer

Colour-changing swim shorts are printed with a thermochromic pigment. Thermochromic is just the technical word for heat-reactive: the pigment shows a different colour when its temperature changes.

On Shift shorts, a splash of water or a bit of heat changes the colour instantly. Cool water switches it back. Warm sun shifts it too, so your shorts can change while you're lying on the sand, not only when you get wet.

What is thermochromic (heat-reactive) pigment?

Most dyes are one colour and stay that way. Thermochromic pigment is different. It's made to show one colour at one temperature and a different colour when it gets warmer or cooler. If you've ever seen a mood ring, it's the same basic idea, just printed onto swimwear.

There are no batteries, no electronics and nothing to switch on. The pattern on the fabric is printed with heat-reactive pigment, so the change happens right there in the print whenever the temperature of the fabric moves.

Why each colourway has two colours in its name

Every Shift colourway is named after the two colours you'll see, cool colour first. The Original comes in Purple to Blue, Orange to Yellow and Blue to Green. The Geometric, with its triangle print, comes in Purple to Blue, Green to Mint and Orange to Yellow. The name tells you exactly what to expect when the colour shifts.

How water and heat trigger the change

Anything that changes the temperature of the fabric can set off the pigment. That's why a few different things all work:

• A splash of water: the colour changes instantly wherever the water lands.
• Heat: warmth, including a warm hand pressed on the fabric, shifts the colour.
• Warm sun: sitting in the sun shifts it too.
• Cool water: switches it back again.

So a day at the beach or the pool becomes a constant back and forth. In and out of the water, into the sun, back into the shade. The shorts keep reacting the whole time.

Try the splash test

The easiest way to see heat-reactive swim shorts in action is the splash test. It takes about a minute and it's a great one to do with kids.

• Start with the shorts dry and out of direct sun, so they settle into their starting colour.
• Flick or pour a little water over one leg and watch that patch change.
• Press a warm hand flat on a dry section, hold it for a moment, then lift it off. Look for your handprint.
• Rinse with cool water to switch the colour back, then do it all again.

If you're giving a pair as a present, the splash test makes a good reveal. Hand over the shorts along with a glass of water and let them find out for themselves. There's a short walkthrough on our how it works page too.

Why the change can be patchy (and why that's the fun part)

People are sometimes surprised that the colour doesn't always change evenly across the whole pair. That's normal, and it's a big part of the appeal. No two swims look the same.

Because the pigment reacts to temperature, each part of the fabric responds to what's happening to that part. A few things make the pattern different every time:

• Splashes land unevenly. A wave, a cannonball or a kid with a water pistol will hit some spots and miss others.
• The sun hits one side. The side facing the sun can shift while the shady side stays put.
• Some areas are wet and some are dry. As you move between the water and the sand, different sections change at different moments.
• Handprints and contact. Anywhere something warm touches the fabric can show up as its own shape.

The result is stripes, splodges, handprints and gradients that come and go.

Care tips that keep the colour change working

Here's the catch with anything heat-reactive: heat is what makes the effect work, but too much heat wears the effect over time. The good news is that looking after your shorts is easy.

• Rinse in cool fresh water after every swim, whether you've been in the pool or the ocean.
• Dry in the shade rather than leaving them baking in the sun.
• Skip hot washes. Keep it cool.
• Keep them out of the tumble dryer.
• Never iron them.

Cool rinse, shady dry, no high heat. For more questions, see our FAQ.

What else to know about Shift swim shorts

The colour change is the headline, but the shorts are made to be worn all summer, not just shown off.

• Men's: quick-dry four-way stretch fabric, a mesh inner liner, and a full elastic waist with drawcord. Sizes S to 3XL.
• Kids': quick-dry fabric with an elastic waist and drawcord, sized by age from 3-4 up to 11-12.

One tip before you order: the shorts run slightly small, so if you're between sizes, size up. Our size guide has the details for men's and kids'.

If you'd like to see it for yourself, have a browse of our men's colour-changing swim shorts and pick the colourway you'd most like to watch shift.

FAQ

Are colour-changing board shorts and swim trunks the same thing?

People search for colour-changing board shorts, thermochromic swim trunks and heat-reactive swim shorts, and they're mostly after the same idea: swimwear that changes colour with water and heat. Shift makes swim shorts with an elastic waist and drawcord, so they're easy to pull on and adjust.

Will they look the same every time?

No, and that's the point. The colour reacts to water, heat and sun, so the pattern depends on what you're doing on the day.

Do the shorts change back?

Yes. A splash of water or heat changes the colour instantly, and cool water switches it back, so you can watch it happen again and again.

What if they don't fit?

The shorts run slightly small, so size up if you're between sizes. We don't offer size swaps, but every order has our 30-Day Happiness Guarantee: if you're not happy within 30 days of delivery, keep them and get 25% off your next order, or return them unworn for a refund.