there is no 64 Mustang, just a early model 65, we like to call the 64 1/2.....check with the experts in my group..
the yellows seem to be pretty rare..but the special order paints are the rarest, they could be ordered with the Thunderbird solid colors instead of the Mustang metalics.....
go to the early Mustang Registry
Do you mean production numbers, or what you do or don't see on the road? Any of the beiges, bronzes, or yellows are uncommon in my area (PA). But I've been the owner of a '66 Candyapple Red hardtop, '68 Wimbledon White Hardtop, '69 Meadowlark Yellow sportsroof (current), and '72 bright lime Mach 1. I've seen them all!