I've heard it called white, yellow, even greenish white, & Spiner himself referenced the makeup as gold, which I always took as referring more to the pearlescence than the color itself, like there was gold colored dust in the makeup or something If the makeup was indeed a consistent color, which it seems like it was, we ought to be able to pin point it, at least in some outdoor scenes under proper light, versus under the ship's lighting To me, in real light or sunlight his color appears to be Cornsilk. Whereas under the Enterprise lights, or general studio lighting, he looks to be either Eggshell or Bone Cornsilk Eggshell or Bone
Apparently the book "Star Trek: Aliens and Artifacts" explains what makeup they used. Here's what a poster on another forum said the book says: Link to post So basically the makeup was a shade of beige with gold powder mixed in.
Along with the effect that lighting has, the makeup likely also has some shading variations to accent or de-emphasize facial contours and age lines.
People were surprised he looked so yellow in "Generations", but that's the main difference with lighting for TV (which made him look extremely pale) and the lighting needed for motion pictures. When I used to perform Data at conventions, I invested in two pots of a creme makeup: blending quantities of white and canary yellow to make a pale yellow. Then I dusted with a transparent setting powder, and then a covering of fine yellow/gold body glitter.
I would say pure white or yellowish/goldish-white. I've also noticed a slight green tint in some places, and in others a bluish tint. The bridge of the Enterprise-D has a yellow tint to its lighting, as a result we think of his skin as more yellow, whereas in engineering, it is mostly yellow as well, with the exception of the light coming from the warp core, which darkens his skin tone a little bit.