Not sure if anyone has had this problem but its happened a few times and im not sure why.
I have been using the Americolour Regal purple and made cupcake decorations in a nice purple shade, 2 days later when I looked it was blue
its not the first time this has happened, any one know why?
This also happened to me with rose pink in buttercream that had milk added.
The purple was stable for me when I added it to buttercream for flowers (shortening, sugar, corn syrup) using an old Richard Snyder book.
So maybe it's the interaction with the calcium or something else in the milk that speeds up the degradation...
I seem to have the problem with purple too. I do not use any dairy in my buttercream. The wilton purple is better but still not great. I have found my purples to take a blue or gray tinge.