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It doesn't match up exactly but could it be Space Goofs?
That looks familiar somehow, so you might be right, but what the heck is this thumb-wrestling thing I'm remembering so clearly? Are there any episodes where they thumb wrestle?
I also remember a Thumb Wars show on Cartoon Network.
No, it wasn't the live-action thumb wars show from the mid-2000s. This was a cartoon that I would have seen around 1999.
Bump - still haven't figured out where the "Battle of the Brains" comes in...
The thumb wrestling thing jgrif mentioned sounds like TWF. Thumb Wrestling Federation.
I figured that out already.
This would have been around the year 2000. Possibly cartoon network but not sure. The show was about three aliens with blue, pink, and green skin. The green alien was a nerd, the blue alien had a big forehead, and the pink alien was the girl. For some reason there was an emphasis on thumb-wrestling, which the characters referred to as "The Battle of the Brains!"
It might have been from the creators of Mega Babies as I remember there was a short film about the creation of Mega Babies right before the alien show where the creators discussed how they invented the series. I also seem to remember it being in a similar drawing style. But it's not Mega Babies as I can't find the all-important 'Battle of the Brains' quote in it.
I've been trying to figure out for years what the heck I watched!