I have a batch of vanilla buttercream leftover from yesterday that is already colored a light blue - can I add melted choc to it or do I have to make a new batch? Thanks!

Yes, you can add melted chocolate. But make sure the chocolate is not too hot!

thanks! ive been wondering the same thing!

It will be ok because its lighlty coloured. If you have a dark coloured bc, I don't think adding chocolate be ok. It might give like black colour.

I tend to view these type of things as "why CAN'T you add it?" Unless you're mixing ammonia and bleach, it's usually a pretty safe experiment.

edited to add: Look at it this way ... if your options are "can I mix it?" or "Do I make a new batch?" then go ahead and mix it. If it doesn't work, you just have to make a new batch, which was the other option anyway.