Heh...Spock always is depicted as having copper based blood, and McCoy always ragged on him for having "Green Ice Water" flowing through his veins

.
That being said, I would go with blue or purplish-blue myself. Those colors make the most sense based on the skin tones Tau are depicted as having, and whatever justification you want to use for that is probably fine considering this is
40k we're talking about.
This whole thread reminds me of an old USMC running cadence:
Blood! Blood! Blood Makes the Grass Grow!
We! We! We make the blood flow!