I'm honestly surprised at how many high votes there are for this, because I'm left wondering why I wasted my time on this. It's sugary sweet (which isn't a problem if you're in the right mood for it) and painfully clichéd, but it has a wonderfully soft, peaceful feel to it. So why did this leave me so cold?
I think the root of it is how unconvincing the whole story is. Usually I can forgive the most unrealistic of plots, but this is just too much of a stretch, even for me. Nanaka, Haijima and Mibu meets at beginning of a new school year when she joins the art club, and I'm supposed to believe that in the space of about 2 days (perhaps a week at maximum), they've all fallen in love? I can buy into the idea of a crush, but that's not it - by the 3rd chapter, two of them have at least professed to love the other and were floundering in the melodramatics of rejections, repressed feelings and whatnot. That is just plain ridiculous. Had this been paced better, I might have been more invested in the characters and their relationships, but the idea of 4 people bumping into each other and falling deeply in love just makes me snort. There's also a gaping plot hole at the end - what happened to the "I have to move overseas" thing, which was the main obstacle? Apparently, it's easily forgotten once the prescribed number of pages ran out and the author had to throw the leads together.
Another weakness due to the short length is the characters, none of which have any development whatsoever. Haijima is quiet, anti-social and bad at expressing himself, and apparently so is Nanaka. Mibu is the playboy who masks his feelings, and Meiko's... just the cheerful best friend. So why should I care about these people and their silly romance? I found Haijima particularly annoying - he's just there to look pretty, does one or two things that the female lead supposedly finds attractive, and POOF! we have a love interest. He's frankly a bit of a jerk, and it figures that the nice guy finishes last.
The only good thing I can say for this manga is that it has beautiful art and a lovely, serene atmosphere. Perhaps it would've been better had the author focused on the art theme and ditched the ridiculous love square? 24 Colors had potential, but in the end it really fell flat. It tried to go for too much and should have been either better paced or extended into at least 2 volumes... but on the other hand, I'd probably gag if this plot was dragged out any longer.