hi alistu,
i suggested you to go through cauchy and sellmier eqution to understand the dependece of refractive index on wavelength(as you asked for that).
now you see wikipedia graph, and you beleive it would be always true for other materials, you can not paint the every thing with same brush.this graph is for BK glass.for other materials the graph could be different.
genneral for produccing negative dispersion we introduce a heavy doping in core so to increase its refractive index, the graph is some how inverse of that.
you might have heard of dispersion slope?
what is that, it is not only we have to cnacel the dispersion but we must have match dispersion slope .
regards