The way I do it is to stick firmly to the brown side of white, rather than the grey side (I do this to keep my World Eaters and Death Guard looks distinct). So for
DG, I'll start with a mix of brown and a cream/ivory colour, and layer my way up to pure ivory, then wash with Agrax Earthshade in the recesses to get the dirty look.
The only pic I have to hand is this deceased
DG on Angron's base, but it gets the point across:
As you can see, the 'grey' is actually a very light brown, or a 'warm gray' really, as opposed to the 'cold' grey that comes from working with blues/light greys.
Hope that's of some use. If not, I'm currently working on Mortarion in a similar manner, so if you haven't got an answer in a day or two I'll pop back with some examples from that.