Explain refractive index for light of different colours.

Explanation

Each colour is associated with wavelength of light.
In the visible spectrum red light is at the long wavelength end (~ 700 nm) while the violet light is at the short wavelength end (~ 400 nm).
Dispersion takes place because the refractive index of medium for different wavelengths (colours) is different.
For example, the bending of red component of white light is least while it is most for the violet.
Equivalently, red light travels faster than violet light in a glass prism.
Table gives the refractive indices for different wavelength for crown glass and flint glass.