Congratulations to …. For the correct answer that the colour of Mr Spocks blood was green. Leonard Nimoy who played Spock is the only actor to appear in all 80 episodes (counting the original pilot) of the series.
Gene Roddenberry the series creator, originally wanted Mr Spock's skin color to be red, but that would have meant many extra hours in make-up for Leonard Nimoy. Luckily for him, initial testing showed that the red color appeared as black on black-and-white televisions. Since most televisions in the '60s were still black-and-white, the producers scrapped the idea and no doubt Leonard Nimoy breathed a huge sigh of relief.
Spock’s makeup had a faint greenish tinge to it, because of his green Vulcan blood. Because the makeup was hand-mixed, the amount of green varied slightly, and in many shots (even close-ups) it was more distinctive than others which is no doubt something that devoted Trekkies would have picked up.