I was making some fondant hearts recently and used some petal dust to color them. Since the dusts don't change color it's good to use a powdered color for any shade of purple, or you'll end up with blue instead.
So yes, I should have used a Crystal Color for this, but I grabbed the first thing I saw. It mixed up and colored fine, and I started rolling out the hearts using the pasta roller.
I did the first group with no problems, and gathered up the excess fondant to re-roll it. Then I noticed that the color in the fondant seemed to be turning bluer. When I looked at it closer it was speckled with blue dots, which I assume was the color in the powder separating out. Stupid powder color.
I had only used half of the fondant for the first group of hearts, and I compared the first half to the one that had been sitting there. Sure enough, the color was changing and getting speckled.
The heart and the bottom piece in this photo are the first roll-out and the part that hadn't been rolled. You can see that the piece that the heart is sitting on looks more blue...that's the part that had gone through the pasta roller a few times.
I was able to roll out the second half and get the hearts that I needed out of that, but I learned that you need to be aware of the possibility of your color changing when the fondant is kneaded. Just watch out, because if you need to match colors you might end up with a slight difference if you work it too much.