Okay here's a tough one.
I have had this problem before and it is haunting me again.
I base ice in a colored butter cream (say pink)
(using wilton coloring)
When I smooth or try and patch to get it perfectly smooth I get color differences. Is there a way to avoid this???
AND... is there a way to fix a cake once you get that blotchy effect with out an air brush machine????
This problem drives me crazy to the point where I hate it when I have to do a cake with colored butter cream.
ANY IDEAS????????

Question...

Does the color difference remain?

I'm wondering if it's a difference between when the buttercream has crusted, and when it's still "wet". If that's the case, it should end up being the same color once the smoothed area or patch has had time to crust as well.