hi alistu,
i had not exact page no. right now, so you could see that lower wavelengths may have large velocity or vice versa.
but if you would have understood the the cauchy formula, that refractive index(R.I) is wavlength dependent.
AS R.i is defined as speed relative to ‘c’. hence different wavelength may different velocity but infinitesimally small, however for longer link it gets accumulated.
for convenience i will attach any pic soon.
with regards