They look pretty healthy to me. It's just a different leaf colour I think.

Is there any sign of yellowing on the new, smaller leaves that I can only see the backs of? I'd nip off the yellow ones if possible - looks almost viral in origin, but time will tell.

Thats funny...I have the same on one of my Dahlias. I thought probably virus, it doesnt seem to be doing any harm. Your Begonia looks healthy too.