I made a batch of Buttercream Frosting using Crisco, (Wilton Recipe) except I took out the butter and used the full 1 cup of Crisco, Wilton Vanilla Flavoring, Powdered Sugar, and water.
Taste fine...loosed fine. Then I mixed the first color in. It mixed at first, then seperated. So I tried Americolor it did the same thing. I tried several different colors.
So I made another batch using Walmart Crisco, thinking it might be that...nope, it did it again.
ANYONE HAVE ANY CLUE???
Are you trying to make a dark color??
This sometimes happens where it seems like the crisco hasn't mixed with the sugar well, try letting it set for a bit and them mix the hec out of it. I believe its the crisco that is preventing your colors from staying blended. It is usually most noticable with the darker colors and usually more so when left in a bowl covered when used on the cake i haven't encountered this problem.
Sometimes if an icing is too thin, it can "separate". This is much more noticeable as peg818 mentioned with dark colors because you end up adding so much color that it thins the icings down. Try adding some more ps and see if that helps in the future. Good luck!
Mine does it when it gets warm. If I hold my piping bag too long with hot hands it is worse.
As was said I have this happen when I am making red and black or other dark colors. When I am making the deep colors I add a little cornstartch to the icing and that seems to help me.