Iro wa Somaredo

Hotori has always tried to be a "good girl" and live her life without making waves. When she forgot to bring the survey book for her teacher Enjou, she tried to sneak back into the classroom at night, only to find a man asleep, surrounded by painting supplies.

"So it was you!"

Hotori ran away when the sleepy man suddenly kissed her. But the next day at school, she is reunited with that man Kodo, who claimed to be the new substitute art teacher... He is not the type you would usually call a teacher, with his impudent and slovenly attitude. At Enjou-sensei's request, Hotori is pushed into modeling for Kodo's paintings......

A pure but inappropriate after-school romance between a lazy art teacher and a "nice" high school girl.

source: https://www.mangaupdates.com/series/u6agrfd/iro-wa-somaredo