I had a bunch of them that match your description last year, but I bought them that way. I'll poke around and see if I can find the name.

Hey Nigella...Just the other day I saw some calladiums that looked like my Mom's. I think it's just the way new leaves appear and then they get their green and purple coloring later. I saw my Mom's the other day also and it still looks the same but the plant is very healthy and grows like crazy, so whatever it is, I am assuming it is normal. Not mutant, lol. Oh also, the leaves were very large and a bit rolled up, so that makes me assume they are young leaves. I do a search as well and see what I can find.