Hey! It seems that the mix tool doesn't work well with the latest Aseprite version (it doesn't replace the template color by the mix).
Viewing post in Magic Pencil for Aseprite comments
Sorry! I just figured out the problem. My pencil color mode was in "Copy Alpha+Color". This mode negate the mix effect (it just draw the pink alpha color of the tool). I don't know if this is a bug or expected behaviour, but there is an easy alternative so it's ok!
Also I forgot to thank you for all your extension and awesome tools for aseprite!