You know there are vintage catalogs listed in another thread, right? If you find a bag you like, and you can see the Creed, that will give you a lot of info on how to estimate when it was produced and in what colors.
But yes, even with all that info, it can be a bit of leg work to get an idea of what color a bag is or what colors it was offered in. I find many older colors change with age, wear, the slings and arrows of outrageous fortune, etc, so it is possible to end up with bag that looks a bit different from what it looked like when brand new.
IMO, all bets are off for neutrals offered before 1993.