Oh wow a visual queue would work perfectly for that! I see what you mean! Maybe squish down a bit like they are ready to spring and get a little darker or something! Maybe even put a little charging up a jump sound effect for good measure.
My idea actually started out as being multiple characters as well! I forget the exact moment it evolved into abilities, but I was always using these colors in the placeholder sprites and I thought "Huh, maybe the character just changes colors with the ability instead" Then I came up with the slime idea because usually they are pretty colorful fellas haha. Don't sell yourself short because your characters had a lot of charm! I really liked your approach!
Thanks for detailing what you meant! I'm inspired so I'll probably work on that enemy mechanic tonight!