For most problems you can assume that the speed of all wavelengths in a certain media is the same. Mirrors, glasses and a lot of other day-to-day objects work with that assumption just fine.
If a more precise experiment is made, it can be determined that the speed varies just a little bit for different wavelengths in the same media. That phenomena is called dispersion. Besides the prism experiment it starts being noticeable in things like photo/video lenses as a color fringe at the edge of the objects.
Another way to think about this: Normally we accept that the Earth is a sphere. For certain more detailed measurements we actually have to account that it's somewhat different from that - it actually being a sphere squished a bit in a certain way. Dispersion is the same way - it's so small that it can be ignore a lot of times but it is there when more precise measurements need to be made.
Frequency does not change. Speed of propagation changes and that makes n change as well.
