Why didn't they just paint the actors blue?

I've just watched a Fox special about the making of Avatar that left me even more confused. What's the point of all that motion capture technology, all that facial mapping, all that laborious cgi recreation, all those hundreds of billions of dollars, if the aliens look just like people? I'd get it it if the avatars were non-humanoids, or even half-humanoids, like Gollum, but they're totally humanoid. Two arms, two legs, one head, dreadlocks, braiding, blue skin. What was so challenging about them? Give me five minutes with some finger paints and I could have saved 20th century Fox $500 million. Imagine if Cameron ever shot a Western; he take one look at the Indians, freak out, and then reinvent cinema just to get their skin color right.