No need to use a wash, you neet to know how to LAYER but more importantly you need to start using intermediate colours for those tricky ones. i.e. Yellow, Red and White.
When using these colours you need to use a basecolour that is closer to the desired colour but has good coverage properties.
Light greys for White.
Orange for Reds.
A mix of Yellow and White for Yellow.
LAYERING.
When layering over detail you need to angle your brush 90 degrees so that the brush only touches the raised areas.
So if we start with a black undercoat on the shoulder pad with Terminator Honors, Layer on Shadow Grey remembering to hold your brush 90 degrees to the surface to be painted. This will leave the eye sockets, nostrils, teeth and the surrounding recesses black. The final stage is then just layering on a few coats of Skull White over the Shadow Grey. Belive it or not with this technique I can get pure white with only 3 layers of white.
Please note: Your paint needs to be 1/1 (1 part paint, 1 part water) or test the mix untill your paint doesnt run into recesses.
Also you might like to look at my Terminator Painting Guide here http://www.spacemarines.co.uk/stage1.shtml It uses an Ultramarines colour scheme.