On a hot summer's day it's easy to recognise that a black surface is often much hotter than lighter coloured surfaces, especially white ones. Many people think this happens because dark colours absorb more heat, but this isn't actually true -- it's the ability of dark colours to absorb light that makes them hotter.
Colours and wavelength
White light is the mixture of many different light wavelengths, and each of these wavelengths has its own colour. When an object is cast in white light it often reflects different wavelengths back, and these reflected wavelengths give the object its colour. If no light is reflected and all of it is absorbed the object will appear black.
Conservation of energy
The law of conservation of energy states that energy can never be created nor destroyed. It can be transformed into different types of energy, but the sum must always be the same.
Light to heat
Because darker objects absorb more light, they retain more energy from sunlight. This energy cannot simply disappear, so the darker object releases the extra energy by emitting heat. Thus the more light an object absorbs, the more light energy it must transform to heat, and the warmer it will get in direct sunlight.
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