So reviving this after some stuff that I saw today when I was playing. I have 3 sims:
A has Extra Creative, is level 10 and all skill challenges. He paints with sparkles.
B doesn't have extra creative, is level 8 and the 6 brilliant painting challenge complete. She paints with sparkles.
C doesn't have extra creative, is level 8 and has no challenges. She does not paint with sparkles.
From this I deduce that either the brilliant painting challenge is the one that influences it, or it is completely random.