I use Alice's cooking icing. I make it with about 5 tbls of water first to outline. I then tint it the different colors I want, outline, and then add more water to each color to fill them. That's when it "separates". White flakes appear. I believe it's the butter. I tried adding hot water this time, but it still did it. What am I doing wrong?
I've had this happen with buttercream when I thinned the icing to fill in FBCTs. The only thing I could figure out was that I needed to beat the icing again instead of stirring the water in. When I thin my icing with a mixer, it never happens.
Bunnywoman, over on the Wilton forums, suggests when using the MRI to flood cookies, reduce the amount of butter to 1/4 cup from 1/2 cup. She has found that this will keep it from separating. You might want to give that a try.