I have the same problem with Americolor violet and regal purple. I think it's the nature of the color, when left out in the air or light it fades. Annoying, though, isn't it? Especially when one is tring to get things done ahead of time.
i used violet on my fondant/gumpaste class cake and it faded to light blue. i havent had a problem with BC the first couple of days but after theat the pink starts coming out of the violet. my instructor says you have to mix colored fondant into the white so that the color doesnt fade
I used to use Wilton and had the same problem. I recently switched to Americolor and still have the same problem. I agree that I think it is just the nature of this color. I also have problems with it "spotting." I can decorate a cake with it one night and when I look at it the next day after it has crusted well, there will be a few dark spots as if the icing wasn't mixed well.
I'm not sure what to do? Oh boy. I was hoping this was not the case. As far as the spotting, I think (hypothesis from my experiments) is that this happens when you add salt to your bc. I found that if you first dissolve the salt in warm water before adding it to your recipe the spotting problem is solved. 
Blue + Red = Violet
When the #2 Red Food Dye was removed from the market over 20 years ago, this is the problem that came about. The reds today are not as stable or solid as the #2 Dye. When in direct sunlight (or some bright lights) the red doesn't hold the color and it fades. When the red fades out of purple, then all that is left is the blue.
No big mystery .... just basic science.
Oh boy. I was hoping this was not the case. As far as the spotting, I think (hypothesis from my experiments) is that this happens when you add salt to your bc. I found that if you first dissolve the salt in warm water before adding it to your recipe the spotting problem is solved.
Actually the spotting can accure even if you don't use salt in the recipe. I have seen this happen with pink and purple more often than other colors. I always ask my students first if they used salt, if they didn't it could be that the crisco or powder sugar didn't get mixed very well and there were small lumps or it could be the water and suggest they try it with bottle water.
If you get these the best thing to do is make your icing ahead of time. Once you have the spots you can just stir your icing and mix them in. Once they are mixed in they will not reappear.
As far as the purple fading I have heard that if you use milk in your buttercream icing that it does help with the fading. But as others said it is the nature of the color. Sunlight and florescent lights will fade it faster than anything.
I remember that when I took my Wilton classes we were told that violet will change if the meringue powder is omitted from the recipe. We were told the meringue powder acts as a stabilizer and will help keep the color.
And the one time I allowed myself to run out and made violet anyway the color turned blue by the time the cake was delivered. I was so embarassed. 
That was 16 years ago and I swore I would never do THAT again.
Sandy
I always get a kick out of the partially faded cakes at the supermarket, for sale in the refrigerator. Um, no, thank you. 
I just took the Wilton Fondant and Gumpaste class. My instructor and I both colored our fondant with Wilton Violet. It faded. We also both used the ready made Wilton violet and it didn't fade. Maybe because of the preservatives in the packaged stuff? I'd suggest if you need violet and don't want to worry about fading, use the precolored stuff. It held the color well.
I remember that when I took my Wilton classes we were told that violet will change if the is omitted from the recipe. We were told the acts as a stabilizer and will help keep the color.
Sandy
I personally think that's a scam to push products in order for them to do more business.
Reason being, when I took classes, I was told that meringue is a stabilizer and it will prevent the colors from fading or bleeding and that it will also cause the icing to crust over. If it was omitted the icing would not crust over.
Well I never take anyone's word at face value, I tested it by omitting the meringue. Sure enough my icing still crusted over (actually it crusted over better than when I used it) and I colored a portion of the icing and left it in the same amount of light as butter cream icing made with meringue. They both faded in the same manner.
Since then I never get my knickers in a knot f I don't have meringue powder. If I have it I use it, if I don't oh well, SHLGO.
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