I read this years ago, but I can't remember the title and searches are turning up nothing.
I remember the main character was just your generic male manga / anime protagonist, but it was largely focused around a young artist girl who was like a child prodigy painter.
I think she lived with a relative or something and was an orphan, and the main guy ends up helping her become a famous artist. I THINK she used a treehouse or something as her studio.
The main girl might've been blonde and had twintails, I'm not sure, but I do remember she was usually barefoot and covered in splotches of paint.
Would be really grateful if anyone could help me find it.

Boku to Rune to Aoarashi

That's the one, thank you!