The Army Painter put up a 2 page .pdf online to help you mix colors into new shades. It’s perfect for helping you hilite your models.
From them to you:
The new Warpaints Range has all the colours you need about 90% of the time. But once in a while you will need a colour not available amongst the 36 Warpaints pots.
Actually, you do not need 18 different nuances of brown. It is your hamster-like hording instinct taking over and your wallet will suffer as a result. By having a good base of colours you can mix any colour imaginable. To help out on the more odd colours, we have produced a small Mixing Guide in pdf where we show you how to mix some of the colours not available in the Warpaints range. In time we will have more and more mixing pages added to the guide for a more comprehensive and complete document.