1) Xbox shells aren't black, they're very, very, very dark green. Think I'm on crack, fine don't believe me, but if you look close you can tell. If you own a black gamecube (I assume a ps2 would work just as well) put it right next to the xbox and look at it under good light and you'll see exactly what I'm talking about.
2) They don't paint the plastic, that's just the color it is. It's just a matter of chemicals. So it's possible this is slightly cheaper to make, but not much if at all.
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