It seems to me that when buttercream is colored it gets testy.

Takes much longer to get it smooth and sometimes can appear downright grainy.

Should I add more liquid to a BC that will be colored?

It almost seems like the color dries the BC out.

I can't say that I've noticed that happening. Maybe someone else can give more insight!

I haven't had that happen either. What are using to color it?

Hasn't happened to me either. Are you using plain old food coloring? I use the Americolor Soft Gel Paste and it works wonderfully.