The colors you see when a rainbow appears are the result of light being split into its various individual wavelengths. This gives us a spectrum of colors that range from the shorter blue and violet wavelengths through to the longer red wavelengths. This sequence of colors gives us the characteristic pattern we're all familiar with, and that we learn from childhood through the use of mnemonic phrases. The colors of the rainbow are Red, Orange, Yellow, Green (in the middle), Blue, Indigo, and Violet.

Source: Met Office