don't forget people see colours differently, sometimes extremely so - and it also can be different between the sexes (I remember the hugest argument that spanned about two days, in one of my jobs, as we were split between whether a lumber marking crayon was yellow or whether it was green - all the guys thought it was green, all the gals, yellow)
Anyway, just my two cents, if you're going for a particular colour - try mixing your own. Keep notes, so you can recreate what you did, and as mentioned above, keep in mind it will also depend on what colour you're going over (base/primer/shadow). It's quite a lot of fun to mix your own colours! And really if you keep notes (ie - one 'drop' of green, one of grey or what have you) you can pretty easily recreate it time and again.
Also, that way when you are trying to remember later how you did a particular colour for your army, you'll be able to find your notes and remember!
Good luck