Red is the problem color. Here's a thread that explains why purple turns blue and why it's been doing that since the 1970's:
http://cakecentral.com/cake-decorating-ftopict-653200-red.html+dye
Perhaps a small amount of black.
If you can find it quickly in your area, the only purple coloring I use is Chef Master No Fade Purple.
Other than that, I'm stumped. Check it out in natural/sunlight to see if it's an actual color problem or just an impression.
Best of Luck
Rae
I made a cake with purple fondant and coloring it was the hardest part. I started with a lot of violet then added red and blue and believe it or not a touch, just a touch, of brown- I added the brown because I was at a loss and thought it just couldn't hurt. It was late and I was beyond caring- It actually worked- it just seemed that I was adding WAY too much color
I had the same problem getting fondant purple. It was purple and then half an hour later it was blue! I ended up changing my design mid-cake! It was Fond-x, btw, which was great otherwise. I had some Wilton fondant that took the purple fine and used it for accents. The next time I was at the cake store, I asked about this issue. The guy told me to use half gel coloring and half powdered pigment. The next time I had to make purple fondant (Pettinice this time) it totally worked! That was a week ago and the leftover stuff is still purple. BTW, I used Americolor gel color and Crystalcolor majestic purple. 
I can guarantee you that your great result was because you used the Crystal Colors purple powder. Those are FDA approved food colors that are extremely pure. They never fade or change color because they're not mixtures of artificial colors. You can use them to color batters and icings, too.
You can find them online from the developer's of the line:
sugarpaste.com
Rae
Was it Fond-X Virgin White? If so, the whitener in it is what is preventing the color from working. And it's not just with purple. I've had problems with it changing the colors of various blues and greens as well. For those colors I have just had to stop using Fond-X.
I have made a couple of purple cakes. I for a lilac color I started with white fondant and used a little wilton violet and a little americolor fuschia. To get a darker purple I started with white fondant, used wilton sky blue, violet, and burgandy. Check out the pics on the purple topsy turvy and the bowling pin cake. My hands were purple for at least a day after! LOL 
Was it Fond-X Virgin White? If so, the whitener in it is what is preventing the color from working. And it's not just with purple. I've had problems with it changing the colors of various blues and greens as well. For those colors I have just had to stop using Fond-X.
It was virgin white. And thankyou for solving that mystery for me. I am still on the search for the fondant I will use. I know it's not that popular, but I'm really liking Pettinice. It takes colors fine, I can roll it thin and it tastes pretty good. I don't want to have to mix brands on any one cake because they all look and act differently. I'm not the OP, btw. I was just comiserating with her about the woes of purple fondant!
Mine was actually MMF, I haven't tried anything else except Wilton Fondant(yucky). My problem isn't with fading, it is getting it purple in the first place. I am making a guitar cake for my grandaughter. First attempt at this by the way. She may just get a blue guitar instead of purple. She'll like it either way.
Quote by @%username% on %date%
%body%
