Mega Charizard Y is likely to have the same shiny colours as normal Charizard because they are pretty much the same apart from of course, their stats. But Mega Charizard X is completely different. Sprites haven't been announced for shiny Megas yet though. The best thing to do is to get a shiny Charmander, and find out!!! :)
Hope I helped!!! :D
EDIT: here is what I think is actually looks like...
So yeah, he is teal with orange highlights and red wings.