Ah...purple. The color of royalty, the Colorado Rockies, and among other things, a dreaded color to try to create in edible mediums. Buttercream, royal icing, fondant, gumpaste - the list goes on...they all have a tendency to fade to blue when colored purple. (Purple, while the most common culprit, isn't the only color that will do this, anything with a red component has the potential; see below.)
Why? Well, there's a lot of science behind it, but basically red + blue = purple, and, due to regulations, the reds that are often used in food dyes today are fairly unstable. Result - the red fades quickly, leaving behind only the blue.
So, how to combat the fade? There are several food dyes on the market that are "no-fade" and seem to give good results, but if you don't want to purchase a special product, try one of these approaches:
- If your recipe calls for a liquid (i.e. water) substitute milk in an equal amount.
- Add a few additional drops of red or bright pink food color.
- Use powdered food colors.
And, keep in mind:
- Lemon juice and cream of tartar will often cause the color to change to blue; save yourself some trouble and omit these ingredients if they are included in your recipe. (The acids don't play well with the purple food dye, in particular.)
- Direct sunlight and fluorescent lighting will speed fading, so try to keep the finished product away from these light sources if at all possible.