You can't fix and stabilise the colour because it's caased by unexposed silver halides which have a very fine layer of metallic silver on the surfcae of the grains. The effect is called print out.

The problemis that as sooon as you use a fixer the milky colour of the silver halides themselves is dissolved and that just leave the very fine silver layer which looks darker initially, as hpo is a silvber solvent that in turn will dissolve if left too tong.