Wow, nice scheme!
Few things that come to mind:
I sympathise with all those gems! I mean, wow, you have patience! I probably wouldn't have done the wing gems in that way (except maybe for the Warlock Titan), I would have painted them as surface blisters that house the Holofield tech.
I also see what you mean about doing the weapons black/red, and while they could look good, it depends on whether you plan on doing this with the rest of the army (i.e., will they all get black/red weapons too?). I don't think a cold white will give you the contrast against the bone really... You could do a bright silver? Or do them in bone/dark blue like the rest of the Titan, and then do the ends in a glow-effect light blue or red maybe?
I think the crisp clean look is fine on the dirty battlefield, as I feel it looks like the Titan has stepped into the area rather than having been in the explosions and such itself. As for overgrown or freshly damaged ground, I think either will look good. Sometimes, a model's base is good area to get colours in that don't fit too well on the model (like green bushes), and sometimes the base is a good area to leave with a high contrast in order to show the model off better.
As for painting, I think that if you start with a light coloured primer (like a grey for example) and then repeatedly wash the areas with thin watery washes of increasingly dark blue/black paint, that you can get some great natural shading and highlighting without needing to spend all the effort on it.
You just put each layer on loosely where you want it, and then either go an do something else for a bit while it dries, or help it along a bit with a hair dryer.
As long as the paints are very thin, the previous layers will show through. So that grey primer you started with will still show through the blue/black at the edges, and 'fade' into the main panel areas; the main panel areas in turn 'fade' into the recesses where darker colours have similarly been washed.
Your shading and most of the highlighting is therefore done for you - all you need to do is use a very thin white to add the very final spot highlights in order to maximise the effect.