I hear ya.. I got rid of all my waterbased paint and switched to laquer based. I took a drive up to my LHS but they didn't have any type of brown or tan by Pactra.. not sure if they make one anyways. So basically I just started experimenting with the primaries. It takes some patience but I got it.
(1 part blue: 1 part orange = 2 parts blue: 1 part yellow: 1 part red)?
I wish I new how much of everything I used. As far as what colors though, I used the following Pactra r/c paints...
Daytona Yellow
Blue Streak
Racing Red
It all comes down to the balance you're lookin for. For instance, if it's lookin too red then you'd want to add some blue and yellow (green). using primaries allows more control of the tone I think. I remember at one point my mix was like that and I added blue first.. from there it started to look like an ugly mauve (sp?) color (too much purple). So I added some yellow and boom... brown she was. A couple drops of black and I had the exact medium brown I was lookin for.
this looks a bit darker than it really is but the tone is spot on the money to what I was tryin to get