Speed of light in glass is dependent on the colour of light as different colours have different wavelengths, thus different velocities.

We know that refractive index of a material in terms of the wavelength of light is

μ = a + (b/λ²)

As the wavelength of violet light is smaller than that of red light,

μ_v > μ_r.

We also know that

Or

⇒ v_v < v_r

Hence velocity of violet light in glass is less than velocity of red light.