Some of Charizard's animations have his wings inward, but overall I don't think visibility should be that big of a deal for most players. However, they seem to be making fighter visibility a big deal in the 3DS game with the lines around characters, and even Alfonzo replacing Toon Link in Spirit Tracks when TL is used (this could just be a aesthetic choice; ST TL seems to always be at the bottom left corner anyway). It makes me wonder if they'd want Charizard to have an alt with somewhat similar colors to his Final Smash. I'm really hoping my prediction is wrong here, and Shiny Charizard does get in, but "Not Shiny Black Greninja" makes me question if we'll get Shiny Charizard.
And for Sonic's color, I couldn't tell much, but the two new Sonic colors seemed to to have a strong body shade for Sonic (mainly the light blue one w/white shoes).
I hope we event get another demo with Charizard playable in the new few months. I'm curious to see if his existing color options have had changes, along with the possible shiny (if shiny happens, hopefully MCX will be green).