What would be a good color to do a wash on a solid sand painted hummer? And what would make a good filter color to apply over it?

Washes are a nice, artistic way to make details "pop out," and are even realistic on rare occasion. Burnt Umber and Raw Seinna are great colors for washes on desert vehicles. Especially ones which rust a bit, which makes them less than ideal for your HMMWV project.
The sand colored paint used matched the actual dust so perfectly the vehicles often looked clean, until we climbed on top and found half an inch of dust covering them. A pinwash on this just wouldn't have the right look:

The only weathering detail on an otherwise very boring HMMWV:

Filters add richness and color variation to what should be a very monochromatic vehicle. This one just arrived back from its daily mission:

So...you can go artistic (which many contest goers do, and there's nothing wrong with that) or for realism, in which case you're looking at a pretty boring finish.

From 18Bravo's pics it looks like pastels-pigments (which ever) and maybe some preliminary drybrushing is what you'll need.
Maybe even a light overspray overall with a base+ white or very light grey would get the correct look.