I would think a pale gray would be ok, but using washes on Real Space vehicles is easy to over-do, if you ask me. For the most part, we're modeling panel joints that are, by necessity, airtight. There really aren't a lot of "deep" panel lines. There are exceptions, of course. Also, we're talking about vehicles that are probably the most meticulously maintained machines around. For many of us who started out modeling aircraft and learned to appreciate an oil-stained, paint-chipped and sun-faded B-25 that flew in the Pacific Theater, modeling spacecraft requires us to alter our way of thinking about what a panel line should look like..
The problem with trying to determine what kind of wash is appropriate on a spacecraft is that you'd have to go by in-flight photos, and the lighting up there plays lots of tricks on the observer and the camera. Those tricks are further exacerbated by the various printing methods used to reproduce the photos that we wind up seeing. Even on the ground, look at how many modelers debate the "correct" color of an SR-71.
One suggestion I might have -- and I'd do this for you, but I'm short of time right now -- is to go online either at NASA's website or try a Google search on LDEF, the Long-Duration Exposure Facility. This was a large experiment carried up on the Shuttle and left in space back in the early '80s. It featured a variety of surfaces made out of different materials. Engineers hoped to stick the thing in space for a few months then have a Shuttle grab it and bring it back to Earth to let engineers see how different materials withstood the exposure.
When the shuttle fleet was grounded after the
Challenger explosion, retrieval of the LDEF got pushed to a lower priority on the missions manifest. I can't remember the exact timing, but LDEF wound up staying in space a lot longer than originally planned.
I mention this because if you can find some good photos of it taken after it was returned to Earth, you might get some idea of how different materials (or at least different colors) "weather" in outer space. That might help give you some guidance on what might constitute an appropriate wash.
Hope this helps. Good luck.
David Hanners
Minneapolis, MN