I don't really like it much.
The characters are all just shonen archetypes thrown into an art setting, but it doesn't have much to say on the matter.
The art (of the manga, as opposed to the subject, I mean) is pretty damn good, and I agree with the previous comment that said that depiction of nudity without an "impure" gaze affixed to it is fresh. It's more matter of factly depicting the subject rather than fan service for teenage boys.
But it's shallow beyond this, the MC is barely more than this tunnel vision kid that is just one attribute with no substance to his character, and I predict that there will be little character growth overall outside of the clichés of their archetypes:
MC will at some point emotionally connect with a subject and probably then see them in an erotic light, resolving a slump he would have fallen into; The lovestruck girl is set up as a loosing heroine: she'll rpobably try to be the aforementionned emotional connect but will then be heartbroken when she discovers the MC's infatuation with the witch, who in turn will open herself to the MC at some point after we learn about her tragic backstory (guessing former teacher of hers that she was in love or in a relationship with, and who died or "dissapeared".) and MC'll help her to move forward.
Annoying rich kid is typical rival and foil to the MC, he'll most likely come around, he's mostly here for contrast. He serve as a mesuring stick for the MC, and will at one point get taken over, rebound and go beyond the MC, who will then get in an art slump. Rival will probably also move beyond "technical" superiority and also surpass the MC in the emotional sphere as well, since he already has a tie here to exploit: his sister. Once he moved to this point the MC will get over his slump and take him over once more.
The sensei is just a typical old mentor, he has ties to the witch's past and it's through him that we will first get clues about her past, he most likely won't be very useful beyond this. The single mother is an instigator of romance, she's rooting for the lovestruck girl and will push her forward in her attempts to get the MC's affection, so little more than a drama plot device. The wild girl is another foil for the MC, simillar to him enouth to drive the plot forward for his developpement as an artist and with her own subplot of conflict with her parents (that's where we are as of writting this at the end of chapter 19), mother probably sick, once initially resolved won't come up again unless mother health take a turn for the worse in the future in an exam/tournament arc and she's forced to drop out. Beyond this she's just a goal for the MC to surpass, like the rival is better in technical execution, she's better in raw expression (and color, we're told).
If the series run long enouth we'll meet a bunch of extremely specialised geniuses and professors in the peanut galery. One of whom will be the "skeptic" type that doesn't see anything special about the MC and will probably serve as an antagonist with a dry and snarky character, think Snape from Harry Potter. And the ultimate antagonist beyond that will probably also serve has a love rival or opposing the romance between MC and the witch, could be the witch herself.
It would be surprising but the author could have the MC end up with lovestruck girl in the end, if we get enouth time away from the witch. But it's unlikely that she'll follow him to uni once the first major arc is done and the MC's accepted and can reunite with the witch on campus. At least the title is not something like "the witch I met that day" or anything similar, or it would pretty much be set in stone from the get go.
All in all I see classic shonen happening with the veneer of art as a subject. You want more in depth thoughts about art as a career in japan and the differents kinds of people trying to be artists? Read Blue period, it's fantastic. Want to read a short manga that focus on art but also on the relationships between the characters? Then try season of alabaster, also really good, and adresses somewhat the subject of eroticism and nude painting.
Got nothing against this manga and I don't mean to damper the mood of those enjoying it, I just don't see it treading any new ground in any new meaningful way, either in the art subject or in the drama... But hey, maybe I'll be proven wrong, who knows?
In the meantime it's off to the runaway kid arc