Red as hell, actually.
As I know the story, Spock was originally going to have red skin (and be from Mars, the Vulcan concept came later). However, in the late 1960s, black and white TV sets were still common, and the makeup would have looked like dark-grey or black - as if Nimoy were wearing blackface - which nobody wanted.
So, they decided to mix up was a yellowish-greenish hue (which is still sold by RCMA as L-N #1, as in 'Leonard Nimoy #1'). This is a bit of a subtle color, so that for the first few episodes, the network's color technicians, who weren't told he was supposed to be different from the rest of the crew, corrected him back to normal human skin tone.