Question: How do objects that change colour in the sun work?

Andrew McKinley answered on 11 Nov 2014:

The main way in which things change colour in the sun is a phenomenon known as “bleaching”. This is when a colour effectively fades – what you are discussing is the way sunlight destroys the molecule which is responsible for that colour.

You’ve probably noticed posters in your classroom have faded a bit, and it is the red colours that fade first, then the greens, and you’ve probably noticed that it is usually the blues that fade last. You will also notice that in the rainbow the colours are in the order Red – Green – Blue. The fact that colours fade in this order is not a coincidence!

For something to be red, it has to absorb blue and green light. For something to be green, it has to absorb blue and red light, and for something to be blue, it has to absorb green and red light.

Now, blue light happens to have more energy than green, which in turn happens to have more energy than red. Therefore, red colours absorb the most energy, so are broken down by the light faster than green (which absorbs the next amount of energy), and then finally the blue dye absorbs the least energy.

A similar process happens with hair dye – for the colour to take it has to chemically ‘bleach’ your hair, and then give your hair the colour of the dye. When you go into the sun, the dye absorbs the sun’s light, and then it fades in the same way as the posters, which reveals the initial chemical bleach of your hair. The natural colour of your hair will also fade in the sun, but this will happen a bit slower than if using artificial dyes.