I'm about to try coloring MMF in the melted marshmellow stage, and was wondering if the color will fade after I add all the powdered sugar. That is, should I color the marshmellow a bit darker than I want it to turn out? TIA!

It does fade a little but better to add more color later than to have to make more white to knead in!

I don't think it fades much at all, but remember that after you make it, it will darken ever so slightly again as it rests.