Hey Wolfen,
I've painted 6mm British Infantry for use with my War of the Worlds models and also some 6mm modern army minis for my Giant Monster Rampage games.
The method I have used is use a base colour and only ever up to one highlight (any more and the mini doesn't look right).
For the faces, I do a swish across the face with a skin colour and then once dry, lightly paint a wash over it.
When painting uniforms etc... I always make sure I leave a line of the black undercoat showing to differentiate the areas of the model.
I have found these things really make the models look rather nice on the table. Remember at 6mm, there isn't a lot of detail anyway, so no need to get carried away.
I'll have some photos of my 6mm minis up on my blog soon, as I've been working on my Kaiju terrain whilst waiting for more bases to arrive (and the Slitering Death and Swamp Dweller armies)
I hope that helps,
- Ben.