Oh, I'm certain if you keep at it, you'll have it mastered sooner than later... say, I know! Why don't I add a little incentive?

If you can manage to turn the color of your hair into one solid shade of black by the end of the week, then I'll let you decide what sort of spell I'll teach you next -- so long as it is within my power to teach it, of course.

[These little spells were fun enough and served as good fodder for his research on this world's magic... but there's so much more he could be teaching her. Things that wouldn't be quite in-character for "Faust" to suggest.]