As Pabs said, It's a white base coat (though new copies tend to have a cream or light yellow base coat), then the mahogany is grain filled, then all that happened was clear coats got sprayed on top. Except Gibson's nitro clear coat in the 50s wasn't all that clear and had a yellowish tint to it to start with, and as it aged it got yellower. So originally, a lot of Jrs wouldn't have looked that yellow at all, but nearly white.
The grain filling after the base coat is important for the look, as it's not a flat all-over colour.
I'd read this article. you can see from the finished pictures that the grain is quite visible as a result.
http://www.manchesterguitartech.co.u...yellow-finish/