Curious why the sky paint took so much mana. He once used objects like leaves to make his first colored paints with no problem. He only ever gets severely drained when he does something absurd. The first time he made a small pond, healing missing parts from lots of Pegasi and unicorn, and making a dragon from blood are the only documented incidents I remember.
So it is unusual that something so simple drained him so much.
I kinda expected the color to be a magic color that changes with the time of day to match or something.
Alternatively, maybe it costs more mana with more distance from materials. Or "the color of sky" which is a concept that we precieve, but doesn't physically exist, was too abstract for his magic?