As described above, I used Goof Off, which is an Acetone-based paint/glue remover solution dabbed onto a paper towel (or q-tip, depending on where the tattoo was). keep in mind the solution WILL remove any kind of paint on the figure (Tattoos, cutie mark, eyes) so be careful around them. The body is colored plastic, so the solution shouldn't hurt the coloring (test on the bottom of the hoof or something first).
As for painting, I used Tamiya Color acrylic model paint (though I'm sure any decent acrylic paint will do). I used plain gloss paint for the mane/tail, and a flat/satin paint for the wings. To get the right coloring, it was trial-and-error. Just mix white with yellow and play with it. I used a very fine tip brush and a steady hand to paint.
Hope this helps!