Sometimes I find that if you are doing very small areas with a brush
you can get away with one of three methods,
1)Dry brushing one colour over another
2)A thin "wash" of one colour over another, or
3)Painting one over another if the undercoat is still wet
Another method I use for larger areas is to undercoat with a
different colour, ie if i want a brownish shade, then undercoat with
red, then the undercoat will slightly change the shade of the top coat.
Although mixing aproximately what you need is probably safest for
something enormous (read B52) the others work for things that are
considerably smaler (Halifax, stirling etc).
Good luck, and happy experimenting.