Daniel, take a look at a colour wheel and the use of complimentary or the 'opposite' colour that will if added to the base colour reduce it's strength or value without making it change colour, for example, adding brown to a yellow to 'shade' it is so wrong as yellow doesn't turn into brown when it gets darker. Instead the colour wheel will show a purple should be added! Try it and see!!
I bet Einion would know a lot abut this!!!
Ther's a very good book called 'Blue and Yellow Doesn't make Green' . It explains the physics of it all.
David