AI've had gray fondant turn purple when made with Wilton black coloring. I read that may be unique to the Wilton brand. Is that true? I'm making an iron throne topper for someone who will be storing it a week before use. I don't want her to pull it out to realize it's become purple!
I always get a green tint with wilton black. If you get a purple hue, use the color opposite purple on the color wheel (yellow..I think) Just a tiny drop will neutralize the purple. I have heard good things about Americolor black and also Chefmaster.
Americolor black is a true black. I have had it work perfectly, no purpleing. In order to use less coloring, it is a good idea to start with a brown colored fondant/gumpaste (made with cocoa powder).
I never use coloring gel to make gray. I just mix a tiny ball of black Satin Ice or Wilton fondant to a larger ball of white fondant to make gray. A lot less messy & no worries about the color changing.