As Mike pointed out, the first two films were black and white, but it was all color from 1962 (King Kong vs Godzilla) to the present. You asked about "earlier" movies, not "earliest," so that's what I based my answer on.
Actually, in the mid-70's (when the Big G was pure kiddy hero) the fins were the body color edged in silver. They've also been edged in white and purple, too, so it's best to check the exact version you are doing if you want to be, ummm..."accurate."
If you're doing the old Aurora Godzilla (or one of the Monogram reissues), the light gray edges are "right" (that kit is based on the King Kong vs G version).
Whatever you paint them, have fun, and let's see your work when you're done!