Any idea on what colours/washes would be the best to try to replicate the regular grey that FFG uses with his TIEs?

Either Vallejo or Citadel would be fine.

It depends on whether or not you're trying to match existing ships in your fleet, or if you're repainting the whole lot. If you're repainting the lot, then just use the closest match you can find and remain consistent with any future repaints. That way your fleet will have a "uniform" feel to it.

I only want to repaint a TIE Silencer to try match the regular FFG scheme as best as I can. I won't touch the other TIEs I have

Ah. It is a quite dark model. Sorry, I can't really suggest a colour match with Vallejo or Citadel. I use Humbrol enamels, and I've only repainted my Scum fleet so far because I hated the khaki/purple scheme they chose to use.

One thing to keep in mind is that FFG has used several different greys for their TIEs.

Here are 4 different shades with a 5th on the decimator so selecting one that will fit with all your other ties is not going to be possible anyway. When I painted my maneuver dial in this post:

I used Vallejo Model Air Pale Blue Grey with a wash of Citadel Nuln Oil. It's fairly close to the bomber but not exactly. If you want shade with no blue tint Model Air Light Grey should be good.

Yeah, I should have been more precise. To me, when I think of "TIE grey" is the one used on the TIE Advanced or regular Core Set TIE Fighters, or at least something close to that. Seeing that dial Pale Blue Grey seems too dark (even counting the Nuln wash), but maybe Light Grey could do the job.

Thanks!

It becomes much darker with the wash but if you just want to use wash to fill recesses not also darken the underlying paint you can avoid it by put on a gloss coat first or go back afterwards and highlight it back up.

The Advance is a very light grey (at least mine is) even lighter than Light grey. White Grey might be closer to what you want.

I'll have to check and confirm (and post a picture) for you when I get home, but I think I just used Citadel's Administratum Grey to repaint a TIE Punisher, and it looks pretty much indistinguishable from the FFG TIE grays.

Not entirely topic, but the effects models were painted Pactra Stormy Sea Blue. The modern color that's the best match is Tamiya's TS-32 Haze Gray.