Thank you.<quoted text>
Generally speaking, refractive index increases with frequency, so the speed will decrease with frequency: blue light is slower in water than red.
That said, it is possible to have the reverse effect near absorption lines in the spectrum. Because of a resonance phenomenon, the index of refraction can decrease with frequency in that situation. This is called anomalous dispersion.
It is also possible for the *phase* velocity to be greater than the speed of light in a vacuum. This happens in some cases of anomalous dispersion. It actually also happens with matter waves in QM. The *signal* velocity will be below the speed of light, though.
The wikipedia article on phase velocities has a nice animation that might help understanding.