I'm thinking of normal dispersion, of course, far from any resonances, say in bk5 glass or water. I thought it might be due to higher-frequency light undergoing more collisions, but I'm not sure. Thanks!
Doesn't the index of refraction of most materials (glass, water, etc.) increase with rising frequency? See picture of diffraction of light in prism in......... So, in both frequency regions, the index of refraction will decrease with frequency.