Purple food coloring is very volatile because of the red in it. It tends to fade to a blue color very quickly unless you do certain things to it.
I like purple as a color, but working with it can be complicated because of the fading. In addition, it doesn't photograph well, and it tends to look different in different lights. So you can match a color perfectly, then get it to the venue and the light there makes it look a lot darker, bluer, or whatever.
So how do you avoid this problem?
Some brands of food colorings are more fade-prone than others, so part of the solution is to just experiment until you find something that works for the applications you want to use it for. I find that airbrush coloring in general tends to keep its color better than paste colors. Your results may vary...
One way to be sure that your color isn't going to fade would be to use Crystal Colors, which seem to hold the color really well, to mix into the fondant or icing. The only drawback to that is that it would use a lot of the color to get a dark purple color.
The solution that I usually rely on for very dark purples is to make a lighter purple fondant or gumpaste as a base, then brush the piece with Crystal Color purple as an exterior color. The powder won't fade out, and if you start with a base of the color you won't need to use as much to get a true version of the color that you want. Doing it this way can emphasize surface flaws, though, so be careful!
Regardless of how you end up doing it, make sure to make purple items ahead of time so that you can make sure they're not going to fade or change color dramatically. It's better to have the thing you're working on fade before you put it on the cake instead of afterward.
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com