I know some colors were different, but for basic colors like black or bright red, is there any difference? It seems that the 91-92's are less expensive.

They are all the same, except 87-90 nose emblem uses a stud through the bumper, and the 91-92 uses tape.

The nose emblem on the 1991-1992 is different besides not having a stud for mounting and location. The 1991-1992 nose emblem is more like a thick sticker/plastic composite material than a metal badge like the 1987-1990 versions.

That explains why they're cheaper then.