I try to match the GW official color schemes, but in the end, I figure as long as it's close enough it's close enough.
For example, my AdMech are Stygies VIII, which in theory is Mephiston Red. My Custodes' are being painted in the "generic Custodes" colors -- solid gold with one shoulder pad red, I think Mephiston as well. But my Blood Ravens are using Khorne Red, and to make sure everything all matches up, I'm just using Khorne Red on everything -- since I have a kind of soup-y collection, I'm matching colors as best I can this way.
So my AdMech are a bit darker than they should be, which I actually like. I actually use Iron Warriors instead of Leadbelcher, so all of my metal parts are pretty damned dark after a Nuln Oil wash -- except for hydraulics, which I use Leadbelcher on which makes them stand out. And my blacks I use a trick someone on another forum mentioned, using German Grey from Vallejo and then washing it with Nuln Oil, which gives you an aaaalmost but not quite black, with black shadows. (I think Dark Reaper or Corvus Black would be equivalent in the GW line.
