What is the order of colours in a rainbow starting at the outside edge?

The order of colours in a primary rainbow is red, orange, yellow, green, blue, indigo and violet. You can remember the order by using the mnemonic, Richard Of York Gave Battle In Vain (ROYGBIV). There are many more colours than we are able to visually perceive and many of them are outside of our visible spectrum in infrared and ultraviolet.

White light wavelengths (all colours) are dispersed as it passes through the moisture in the air after a rain or through a mist of water. Refraction splits the light into 7 wavelengths. The colour order in a primary rainbow is determined by wavelength. In the visible spectrum red has the longest wavelength, violet the shortest, so red appears on the outside edge of a rainbow and violet closest to the center.

When light is refracted twice, a double or secondary rainbow occurs and the order of colours is reversed with red on the inside and violet on the outside.