I heard of a baker who did a cake that had a very specific color theme, including purple. They made no effort to make sure that the color didn't fade, and when the bride complained, the baker responded with an "oh well, too bad, purple fades." That would be the wrong reaction.
Red dye is very volatile and disappears quickly, depending on what it's in. Putting purple food coloring in things will often result in blue, not purple, once the color settles down.
Bakers should know this, and it's our job to make sure that the purple color stays purple. Brides don't know this, and it isn't their job to know it. If we're supposed to be the experts, we should be able to handle something like this.
I fixed this by dusting them with a powdered food color, which won't fade out because it just sits on the surface of the dry gumpaste and doesn't react with anything.
Some tips for dealing with purple:
1. Apply it to the surface, don't mix it into the icing, fondant or gumpaste. If you do mix it in, expect it to fade.
2. Powdered color resist fading better than liquids.
3. Test beforehand to see if the color will fade. Make some icing and let it sit to see if the color will change before putting it on the cake.
Kara Buntin owns A Cake To Remember LLC, custom wedding cakes in Richmond VA