I wish I could provide you with specific colors that would match OOB units, but I don’t have experience with simply base coating. However, you could take a quick trip to your local craft store (Michaels, Jo-Ann’s, Hobby Lobby, etc. for the US) and check out their craft acrylics section where they sell acrylics for $0.75-$1.49 a bottle. You could eyeball it and approximate which colors you’d need.
For example, the Russian OOB units are maroon. At the store they’re probably going to have 2-5 shades of maroon, so you could get a few you think are pretty close. If none of them are spot on, mix until it matches. The paints are inexpensive enough that I don’t imagine you’d end spending more than $10 to base coat all your HBG units. (If you end up doing this, I’d avoid the cheapest of the cheap acrylics - you’re already on the low end, but there’s a significant difference between the $0.75 and $1.49 paints. I’d also recommend that you buy gray Krylon spray paint primer from Home Depot or Lowe’s, since that will make the paint stick better and save you time on your base coats.)
That’s my $0.02 - wish I could be more specific in helping!