Look at this amazing little palette from Make Up For Ever called the Twelve Flash Color Case. It's pricey ($93), but imagine the possibilities. You can quadruple (or more) the potential of any one item currently in your makeup bag. A red lipstick can be turned plummy with a bit of blue. Or pink with a bit of white. Or autumnal-orange with a bit of brown.
Each color can also be used by itself for a shock of pigment.
I think this might be my next splurge.