A bit of an odd situation, but the black and white portion is the first part of a two-part story, and the colored part is a sequel to them. However, in the tankoubon the colored part comes first, followed by the first part of the story. Therefore, we’re presenting them in that order.
Kazuki and Wakaba are childhood friends who have long been hooked on mutual masturbation, but haven’t actually had sex. Kazuki thinks that the strange, partly sexual relationship they share prevents Wakaba from finding a boyfriend, so he intends to tell her they should stop their practice.
Chapters 1 and 2 from Igumox’s tankoubon “Boku dake no Yuuyami – Only My Twilight” (僕だけの夕闇); the first chapter is original to the tankoubon, while the second chapter was originally published in COMIC HOTMiLK 2013-08.