See here:
http://www.baka-tsuki.org/project/in..._Illustrations
color 1: Suzuki Mitsuki (Suzuki Souta's big sister)
color 2: Momo denying that she said that Azuki was interested in Naoki (in v3 when she was half-drunk)
color 3: Momo, Azuki, Murasaki wearing Murasaki's cosplay outfits in Murasaki's house
color 4: Azuki and Naoki taking a color print (those small photo stickers; don't know their names in English) on their date (shop has cosplay facilities)
pg 15: Momo depressed about the possibility of Souta having a girlfriend
pg 41: Naoki and Midori walking to the station together after school
pg 55: Akari barging into Naoki's room
pg 85: Naoki (& Momo - not shown) meeting up with Azuki on their way to Murasaki's house
pg 115: Naoki's imagination while the trio were changing (into cosplay outfits in color 3)
pg 145: Naoki & Momo having a big fight over the matter of verifying whether Souta has a girlfriend or not (Momo thinks Souta lied to Naoki, while Naoki was fed-up that Momo didn't take his word, among other things)
pg 161: Date with Azuki (initiated by Azuki in v3) for yuri movie
pg 211: Azuki sending Naoki off with a smile when he had to suddenly leave during their date after hearing about Momo joining a gokon (something like a group date, wiki or google it) with other classmates and learning that a 'dangerous' guy is there as well
pg 221: Naoki catches up to Momo and due to circumstances, mimics a scene that Momo has written in her attempt at doujinshi and tries to 'rescue' her
pg 251: Naoki & Momo joins up with the Suzuki siblings in karaoke, verified that Souta has no girlfriend, and Momo sang one of the favorite vocaloid songs of Souta