You do that by holding swatches of known colors and calibrating that way.

If you have say mixed race models or models of a race you're not familiar with, it would be worse than useless. Somebody with east asian features but western european skin tone would throw off an entire movie.

Yes, but it's not as though western Europeans all have exactly the same skin tone either - you can still make the wrong assumption. I think the idea would be to have multiple people with different skin tones so that you can check that they all look ok.

Even when it isn't what the lab guys were expecting:

http://www.startrek.com/article/trouble-keeping-her-green

Reminds me of an older nerds comment about the NTSC Color TV standard .

AKA 'Never The Same Color'

The techs at the TV station would adjust the transmission so skin tones 'looked right' and left it at that.