I don't think it's known for sure. Mason went out of the locomotive business early and I'm not sure if all the records survived. Mason liked green and a lot of its production during the 70's and into the 80's was green, so that's a likely starting place. Otherwise, brown overall was not unknown during that period as well. I think the use of red wheels had disappeared by the mid 1870's, even by Mason, so those were most likely the same base color as the rest of the locomotive. Striping and scrollwork would've most probably been mainly gold, with limited amounts of other colors. Boiler jacket was russian iron, meaning some shade--it varied a bit from batch to batch--of medium to dark grey.
In short, you're left with an educated guess, but a pretty good one.