I've used an airbrush to get a darker color on MMF. I like the way it looks, but it always leaves streaks on the plate, colors lips and teeth.

Am I missing a final step? Is it unavoidable? or can I do something to get the MMF to absorb the color better?

The same thing happens when I paint with food coloring. I use Americolor airbrush colors. Do the same rules apply when painting or airbrushing on color?

I don't know the answer but I'm interested because I've got recently aquired an airbrush.

How far in advance are you airbrushing the cakes?
Do you live in a humid climate that may prevent it from fully drying?

I've read that some people go over their cakes with a clothes steamer.

Does that set the color better and fix the problem? or does it just make the cake shiny?

Unless you buy premixed fondant in the shade you need, it's just gonna happen. At least it's not as bad as colored butter cream. Yuch!

Coloured fondant will also colour people's mouths, teeth and tongues. It's the nature of the beast. It's actually something I warn brides and grooms about - you don't want your photos showing coloured grins, right? Lol

True, but it's the lesser evil of all the options there are. Yuch to all of them really.