Andy,
colours are as follows:
For my 1979- present day stuff.
They are undercoated white, then a thinned Humbrol Khaki Drill 072 which is applied as a thick wash allowing some natural highlighting where the wash is thinner. Once dry I use Mig productions Dark Wash which I leave on for 10-15 minutes then remove with a brush dampened with clean thinner - the idea is to push the dark wash into the nooks and cranies and lift it off the large flat areas. Once this has dried I then drybrush with the base colour lightened with a touch of white.
The early 1967/73 stuff.
Again a white undercoat with a thinned colour wash of a colour made from 85% Humbrol 083 Ochre + 15% Humbrol 117 US Green and a touch of white to taste. In my case based on a 17ml tinlet (Humbrol) 11.9mls of 083 + 2.1mls of 117 and 2.8mls of white (I know that is not 17mls in total )
The idea is to have the colour thin enough to stain the white undercoat, but not too thin that it does not cover high points.
Once the base coat is dry a thinned coat of Mig Productions Dark wash, left on for a few minutes then 'brushed off'.
Finally a drybrush of the base colour with a little white added.
I alwas drybrush with the original base colour lightened, using a different colour does not look right to me.
There is further information on this site, I've modified my methods from those mentioned here for single dark colours.
http://www3.telus.net/Ritterkrieg/paint.htm Hope this helps?
Andy