[OT]-ish : Figure painting question

Okay, let's just say I've got some figs painted, 
but they lack a certain shading/depth. I'm 
happy with my work on the camouflage, etc. but 
I think what I need is a nice wash to pick out the 
depths. 

Only..... I'm scared. :) 

I've used a dirty brown wash on brown and tan 
figures to good effect. I've used a black wash 
on grey-black figures to good effect. But now, 
having a base colour of sand/desert yellow 
(with green and brown and wee black bits), I 
have no bleedin' idea what to use as a wash. 
I'm afraid brown or black will blech-ify (yes I just 
invented the word) the yellow/sand based camo 
but I can't see how else to get the highlighting. 

I've had one less successful wash venture: UN 
white armoured figures, tried washing with mid-
grey. Not dark enough to be effective. 

So, any thoughts? What's the usual mix ratio for 
washes and do you usually just slap on lots of 
the stuff and let the dilute nature of the solution 
wash the pigment off the raised areas? Or do 
you use a cloth or wipe and take off any 
pigment that settles high (or dry brush over it 
later?). 

Thanks for any advice. 

Tomb Raider. 
Working on figs for Campcon on June 1-3 in