It seems that Reimu is not really in love Yukari in this doujin. She's just being "someone special" to Reimu. Hence Reimu said "not because of like or love", "It's because, I think the colour of your eyes is beautiful." and also notice the narration in page 36 "Just for now, let me have the red I can never catch" it means that Yukari did aware of how Reimu feels (She can reads mind afterall). Maybe Reimu was not sure about her own feelings towards Yukari, maybe she prefers Marisa? Judging from the author works, they also ship Marisa x Reimu. Well who knows? That's just my theory. Feel free to argue with it maybe there's something I missed.
