In the series i'm collecting i tend to get the nicest looking ones i can find, but if a key coin turns up, they all get forgotten in an instant and the key coin (if i have the cash for it) is the one i buy. Even if the key coin might look out of place tone wise you should not worry about that. 1) Once you have one you will see others, if you was still looking for it then you wouldn't see any as good (Sod's law as we say in the UK) 2) It can work as a Key date hole filler and should another turn up then you can part exchange the one you have for the one you want. Same coin, might be the same grade, might just be more colourfully toned whilst the rest of your set is white.