Im having a terrible time with my purple buttercream again. Its fine when I ice the cake but the following day it has either faded or turned to some shade of blue. Ive switch brands of paste colour, from wilton to americolor. Ive also tried using milk in my buttercream instead of water. My recipe is half shortening half butter.  Should I customize purple by using blue and red instead?? It doesnt photograph well either, always looks darker. Any help would be apprieciated!!

I always add a tiny bit of corn starch (corn flour) to my purple and pink fondant to stop fading.  Not sure if that would work with butter cream.  You could do a test portion.  

I read on a blog recently, don't remember which one, that she puts a little dark pink into her purple so it fades to just the right shade of purple (again this works with fondant-but you could try it with buttercream).  This would probably work better than cornstarch for buttercream.

And, of course, keep it out of any light.

It's the red part of the purple's and pink's that make it fade. So, I don't think mixing red/blue would work- but, hey, you never know.

I always add a touch of green when making purple buttercream. Green counteracts blue. As a hairstylist, I have to know these things, but it is helpful in these situations lol

Oops!!  Not corn starch!  Baking soda...duh...sorry bout that.  Just the teeniest amount of it.  I use like a 1/4 tsp in a big ball of fondant.

Powdered colors also tend to fade less, and there are definitely liquid brands that fade more than others. It's just a matter of experimenting to find out which combination works, but purple is a tough one no matter what you do.

My other post disappeared. Orange counters blue, not green. Sorry!