I have my fondant done but want to add more color, could I use a mixer? I am using MMF.

In my experience, I can add color with my kitchen aid before I add the powdered sugar. After the mmf is already complete, I have to knead the color in by hand. hth

Like ajwonka, I'd color before adding the powdered sugar. After the mmf is done, I'd knead any additional color because I'm afraid the mmf would be too thick for my mixer, even if using the dough hook. If the mmf is already made, I would grease my hands with crisco to help keep them from staining and kneed the color in.
One other idea would be to make a second batch of mmf, but make it a much darker shade of the color you want, then kneed the two mmfs together.