I've heard it was a story about Orions being re-coloured from dark green to flesh pink, while Spock was supposed to be red, but it turned black on b-n-w TV screen, so they made him unhealthy yellow pale- which looked lighter than humans in b-n-w and more yellowish and still alien in colour. In any case that doesn't really matter, he's supposed to look different from humans and I enjoy painting him green.