Not sick, just starting to develop and show their adult coloration. Mollies have been bred to a wide assortment of colors but not all breed true. Add to that the probabability that your B-I-L's stock was not from a pure line and it's entirely possible, indeed likely for them to produce young of various colors and patterns.

BTW: Solid colors; sometimes dark, sometimes light is Nature's way of providing baby fish with protective coloration, particularly those that develop flashy colors and markings in adulthood.