Just sayin' don't agree. I think it's an amazing rendition of a character - especially being able to see the eye expression. It's not like Vader where the eyes are masked - the audience needs to respond to a face. And not somewhere at the back of their minds dismiss the character for looking like a cartoon. As for the color - IMO that's an ugly shade of reddish purple, but maybe it's supposed to be.
Quick test:
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Real fire background and a more "classic" purple IMO is pretty distracting....
You can darken it, lighten it etc. but it's still purple vs. what the trailer shows. Like grape Koolaid which isn't going to be taken seriously.
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IMO a true purple works in a space setting with lots of dark and cool colored backgrounds. The light is different. It may be that it'll be in space we see a more purple Thanos.
*Shrug* Don't think the Russos are afraid of color or Thanos' helmet etc. but am sure the decisions they are making are based on lots of screen tests - what works, what doesn't. For a character who needs to be a real ruthless badass. It's not like Hulk and Gamora are flaming kelly green in the films either.