I would look at the Americolor Regal Purple gel color. To me, it looks close enough to the purple that you need. Make the butter cream a day or two ahead of when you need to ice the cake, as the color will get deeper as it sits. If you need to darken it, you can add a touch of black to it.
Yes, as ypierce said, adding the color at least 1 or 2 days ahead should bring out the deep color you are looking for. If you do add black be very! sure to use the tiniest bit - a little at a time. Again, dark colors take time to 'develop' and adding black can cause it to go too dark or totally black if too much is used.
Also keep it out of the sunlight or it will fade to blue, per my experience.
If you have an airbrush, that might be a better choice to achieve a deep purple. Start with a medium toned buttercream and finish with the airbrush. They way, people will not have such purple teeth, lol!
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