I know mixing several colors together gives you gray/black, but if you have purple already I would try adding a little brown first and see what you end up with. Keep adding different colors until you wind up with gray and then if it is too dark, add more white fondant until you get the shade of gray you need.
I had the same problem. Lighting was a factor for me. Under the flouresent (sp?) kitchen light it looked purple; in my living room, it looked more grey. The longer it sat, the more grey it looked. I also had better results using a different brand (Kountry Kitchen) vs. Wilton.
HTH!
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