I ran into this as well, and really never found a decent, definitive answer.
I'd say just eyeball it, from various color photos/color plates (color photos are probably best, if you can find some decent ones), and use either something like the Vallejo paints, other military modeling paints, and/or even craft paints.
You may have to thin the latter a bit to avoid filling in any panel line detailing.
I'd probably go with a nice, light, modern, RAF Sea Harrier Gray. Can't recall what Vallejo calls it.
Another option would be to look in Google pics, to see if there are any images of Russian Camo. for jet models, and see if they list any color suggestions for Russian Grey.
I've been known to print out color plates, take them to the craft store, and sit in the aisle with them to try to match the colors, by opening up the bottle tops. So far, I haven't been admonished by anyone, and it's a lot easier doing that than trying to guess a color match through those rather opaque plastic bottles.
The crapshoot with this system is that every printer manufacturer's settings are probably different, so you may or may not be getting the true color on your printouts, but it's close enough for government work, in my book. Besides, ambient lighting and weathering can change the actual colors a bit too.
I wouldn't recommend trying to match to the Vallejo paint swatches, since those don't match the actual paints in their bottles, unless you splurge on the ones they've made from actual paints. Of course, if you already have some of their paints, you can make your own swatches on a piece of paper. I've been known to do that too, in addition to mixing my own paints as well.
Given the scale, lighter is usually better, so as to be able to see the miniatures better, since in that small scale, they don't reflect a lot of light.
Let us know what you decide on.
