You could always buy some black and white paint using small amounts of white to lighten the tone of the black.... :wink:
No true gent would do that. Sounds most unsportsman like to me and dangerously close to manual labor.
Really: Plynkes has a point but more than that I have very short memory. One batch of a mixed shade on the pallet tends to dry up before you are completely done with it. They you have to spend time getting it right again. Even more frustrating is to get an number of shades just so and then hen you try to get the formula right for another session it ends up looking too harsh.
So I think there is something to say for buying ready mixed shades. I certainly have a kit for skin colour put together from various brands.