What is a Rainbow
A rainbow is caused by the suns rays hitting droplets of water in the sky. They also form in the mists of waterfalls and fountains The water droplets act like a prism causing the light to split into all its colours.
These colours are always in the same order which is:
Red, Orange, Yellow, Green, Blue, Indigo, Violet
Sometimes you will see the colours reversed but this only happens in a double rainbow where one rainbow runs Red down to Violet and the other above it runs Red up to Violet.
Why are they always red to indigo?
When the suns rays enter a rain drop they are bent downward slightly before being bounced off the back surface of it. It then gets bent upward slightly as it leaves the raindrop. This bending and bouncing of the light causes it to separate into its individual colours which causes the rainbow to form.
The reason the colours are always in the same order has to to with the wavelength of each colour. Red had the longest wavelength and so is bent the leased and indigo has the shortest wavelength an so is bent the most.
Each colour that we see is emitted from a different raindrop not the same one even thought each raindrop will emit all the colours. This is because only one colour coming out of each raindrop is at the correct angle to hit you in the eye. This is also the reason we see an arc.