How do I mix in a colour to my buttercream icing without making it curdle? By the time I get the right colour/brightness I find the consistency of my icing has changed.

What kind of coloring are you using.

If you are using liquid...that would definately change the consistency. It would be like using color and water in a way.

I use a gel/paste color.

Yes, I use liquid colours. I will try a paste next time. Thanks