The speed of all colours of light is 3 x 108 m.s-1 in a vacuum.
This is also the maximum speed of light and has the symbol c.

Colours of the rainbow from highest frequency to lowest:

However, when light enters a material medium, the speed is lesser and different for different colours!

In a vacuum, all different frequencies of light have the same speed.
In a material medium, different frequencies have different speeds.

This is confirmed by refraction.
Violet light (shortest wavelength) is refracted more than red light.
Violet light travels slower than red light.

When light travels from one medium to another:

Thus a light wave is better described by itís frequency, since the frequency stays the same whether the light is travelling through a vacuum or through a material medium.

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