Hm, if we rule out the possibility that aliens entered your kitchen and caused this bleeding with their anti-icing-rays... I'd go with Julia and vote for the white color you have added.
But it could also be the other colors. What brand/s do you use? Or maybe the reason is the powdered sugar? The only times I have experienced color bleed was when I used the cheap powdered sugar. It is more corse / not as finely ground and I have the feeling that those large sugar crystals don't only clog tips, but also result in less color stability. But I could be wrong, of course