When do the different colours of white light show up?

White light is actually a mixture of various colours of light. Each of these colours has a different wavelength and frequency to the others. Because these colours have different frequency we can separate them out by passing white light through a prism ( a triangular bit of glass or clear plastic). The separation happens because each different frequency, and therefore colour, of light gets refracted a different amount by the prism.

When we get a rainbow in the sky it's because the rain drops are acting like prisms and they separate out the colours in the white light coming from the sun.