Do darker colours block more sun?

What the research says

Australia’s radiation protection agency, ARPANSA, states it plainly: in general, darker colours and higher dye concentrations absorb more ultraviolet than lighter colours. SunSmart, the sun safety programme in Victoria, gives examples. In the same type of fabric, black, navy and dark red absorb more than pale colours such as white, sky blue and light green. The reason is simple: many dyes absorb ultraviolet as well as visible light.

A 2008 study by researchers at the University of Otago and ARPANSA measured 175 fabric samples to find out which part of colour mattered. The answer was depth, not hue. How dark a fabric was explained how much ultraviolet came through, while the colour itself, blue, red or green, mattered far less. The darkest fabrics let the least through.

Why structure comes first

Dye can only absorb ultraviolet that meets a yarn. It does nothing for the ultraviolet that passes straight through the gaps between yarns. That is why ARPANSA puts the density of the weave at the top of its list, and why SunSmart says that if you choose a light coloured fabric, its structure becomes even more important.

The shop study we describe in does a T shirt block the sun makes the same point. Among 236 summer fabrics, the colours that most often reached the recommended rating were black and navy, but also white, green and beige. Colour alone did not decide the result.

Colour also changes with time. ARPANSA notes that after many washes a garment may gradually fade and wear, and protect less, so a dark piece that has faded is no longer quite the colour it was in the shop. Our guide on how to care for clothing made to block the sun covers how to slow that down.

What about white?

White is a natural choice in hot weather, and a white fabric can still stop a great deal of sun if it is tightly made, thick enough and in good condition. Some fabrics also carry ultraviolet absorbers or optical brighteners, which ARPANSA notes can raise how much ultraviolet a fabric absorbs.

The weak point of white is the thin, open tee. Hold one up to the sun and you can often see light through it. SunSmart’s guidance on uniforms suggests exactly this check: if a fabric lets a lot of light through, it may let a lot of ultraviolet through too.

Does a dark colour feel hotter?

A dark fabric absorbs more visible light as well, and its surface warms more in the sun. Whether you feel that depends on the cut. In a 1980 study published in Nature, researchers measured the heat reaching a person standing in desert sun in a loose black robe and in a loose white one, and found it was about the same. The extra heat taken up by the black cloth was lost to the moving air before it reached the skin.

Close fitting dark fabric is a different case, because there is less room for air to carry that heat away. So the answer depends as much on the cut and the fabric as it does on the colour.

Our colours

Daybreak, our first collection, comes in White, Sand, Navy and Black. Within each piece, every colour is made from the same fabric, so the structure stays the same from the palest colour to the darkest. That is why we start with the fabric, and choose the colour second.

You can see what colour does with two UV cards. Lay the White Sun Tee and the Black Sun Tee, or any two shirts of the same fabric in different colours, over the cards in full sun, and do the shirt test. Every Rezzella piece is made to block the sun. Our fabric goes to ARPANSA, Australia’s radiation protection agency, for testing to AS/NZS 4399 before the first run. The full report goes on the shirt test page.