I really wasn't expecting this chapter to turn into a Whodunit. But since it did, I'm going to lay out the motives.
Mine - Her plan was to socially isolate Isanuma, so that Isanuma would have no one left but her to turn to. This plan failed as of last chapter due to Kudou's interference. She was cold to Kudou this chapter, so her taking steps to ruin Kudou's plans is likely. She also wears the same dark socks the girl carrying the paint bucket was wearing.
Kudou - While her initial plan was to use the photo she took of Kurumi to stress Naoi out enough so Naoi'd agree to a date with her, if Naoi didn't show up, Kudou may have resorted to Plan B. Which is to socially tank Naoi enough that Kurumi would in Kudou's mind have no choice but to cut ties with Naoi due to peer pressure. Kudou also wears the same dark socks the girl with the paint bucket is seen wearing.
Naoi - She's struggling with keeping her anger under control. Rather, she's trying to suppress it entirely, which obviously isn't working as she has no healthy coping mechanism to vent stress. All the things with her family and her insecurity over Kurumi abandoning her were likely amplified this chapter when Kudou sent Naoi the photo of Kurumi having fun with their classmates, especially Kokoro, while she wasn't around. Rather than meet up with Kudou, Naoi may have deemed it slightly healthier to take out her anger on an object instead of a person. Alternatively, Naoi might be trying to self-sabotage and force Kurumi's hand, basically make Kurumi abandon her before Kurumi decides to do it on her own. She also wears dark socks.
Kurumi - She's struggling with being separated from Naoi, even starting to fall into the same mental space as Kokoro and her mother. While she is aware that wanting to help someone can lead to harm if taken in the wrong direction, she also made it clear she doesn't just want to leave the current situation alone. Her desire above all else is to be with Naoi. The paint might be Kurumi's way of trying to reconnect with Naoi, similar to the things they did together in volume one. Plus, Kudou has been using the festival preparations as a tool to keep Kurumi away from Naoi, which I think Kurumi has caught onto.
Isanuma - Accidental. She was the last known person to be alone in the room with the arch. It's possible she was trying to finish the behind schedule signboard as a surprise for everyone, but things took a turn for the worst and she dropped the paint bucket while working. If true, she won't confess to doing it, since she'd be worried about the class disliking her again. Plus, if the prime suspect is Naoi, then it'd be easy on Isanuma's conscious, since Kudou already convinced Isanuma that Naoi's a bad person.
Though I have to wonder if someone could make that much of a mess and not get a single drop of paint on them, so it's possible there could be a lingering clue. My guess for the culprit is Mine, since it feels like direct retribution for Kudou ruining her plans. Who does everyone else think did it and why?
Also, for Kudou's actions this chapter, I don't buy her harsh words at the hotel. Chapter 20.5 contextualizes Kudou's actions quite a bit. I think Kudou was doing what she thought Naoi did to her in the past, breaking her down until she has no choice but to become hers. Whether Kudou is the culprit or not, I could easily see her trying to turn things around in her favor. She clearly wouldn't want Kurumi to side with Naoi during the incident. So, Kudou might provide Naoi with a fake, undesirable alibi by claiming they were indeed at a love hotel together the night before. Problem is that due to Kudou's face injury and what she told Isanuma about Naoi, Isanuma would likely think Kudou was being forced into covering up Naoi's involvement, leading to yet another fight.
And finally, this new incident could very well test Kokoro's resolve to support Kurumi as a friend. This is especially true since Kokoro worked on the arch together with Kurumi this chapter, so there could be some sentimental value for the arch there. If Kokoro thinks Naoi is guilty, this might be when she finally blows up with the negative thoughts about Naoi that Kokoro managed to hold back in chapter 10. Depending on how Kokoro reacts, Kurumi might have to make a serious decision about their friendship and if it's possible to maintain it with Kokoro's unrelenting distain for Naoi.
Chapter 24 is currently scheduled to be in next month's issue of Yuri Hime, but it tends to get an unscheduled break at this time, so hopefully it'll be in the finalized list for next month.