It took me many years to notice that, in Journey to Babel, when Spock's blood is being tranfused into Sarek, there's a little bottle in the line that's removing the human blood elements. I know they SAID they could do it, but I'd only recently noticed the actual human blood being removed.
D.C. Fontana had some clarification about this in her script for "Journey to Babel:"
Scene 73 INT. EXAMINATION ROOM - CLOSE - SPOCK
He lies on an examination table, bare chested. With his look, CAMERA PANS to the device on his arm. From this device CAMERA FOLLOWS transparent tube carrying his green blood to a wall computer.
CAMERA PANS to a second computer, follows the tube from it to the Jefferies Separator. HOLD on the green Spock blood entering the separator, and orange portion dropping down to a bottled labeled 'Waste Factor', CAMERA NOW FOLLOWS another tube which carries blood of a brighter green hue to a device on Sarek's arm. Sarek is anesthetized.
When ultimately realized, the shot looked like this:
In looking at the Jefferies Separator unit, you can see the ochre-green blood from Spock, the orange "waste factors" in the little collection bottle, and the emerald green blood going into Sarek.
So, Spock's blood is green (with a little bit of orange in it), but a pure Vulcan's blood is even greener.