Americolor sent me this email back - someone said to try Hobby Lobby colors so I might test those out. I mix my icing well so I don’t know why it wouldn’t be dissolved.
Hmmm . . . I suppose if it's anything, it's undissolved coloring, not other ingredients. Powdered sugar dissolves nearly instantaneously unless you've got clumps in it.
As a case in point, here's AmeriColor Wedgwood Blue in a bowl, with pink spots on top. I mixed this bowl of icing three days ago, and iced cookies with it. The color set reasonably stably with no pink spots, though not perfectly. On day 2, after presumably enough time for "my ingredients" to dissolve, I returned to the icing to find pink specks all over the top like you see here. I then stirred it to remove specks, and set it aside another day; then I returned today to find the specks again today. This is why I say it seems like coloring not dissolving, not the ingredients. I don't mean to disparage AmeriColor (I love their range of hues), but I just haven't gotten this speckling with any other coloring. Now, this icing was relatively thick (perhaps making it trickier to dissolve the coloring and ingredients), but, again, never this spotting with Chefmaster, which is the other brand I use a lot.