Okay, here's a theory. I think the colors of the collars have different meanings. Back in Dark Alter, the girl's collar seemed to amplify emotions of desire, and in combination with a number of enchanted orbs in the room, manifested into a masculine being. Looking at the color of her collar, I'd say it could be Magenta, which is supposed to be the color of Harmony and emotional balance. I think that the girl in that story was an enemy combatant with some sort of magical ability, who was forced in that room to subdue her desire to fight by manifesting her desires to submit creating balance.
In Alison's case, her collar being blue might represent trust and loyalty. Torrin is the first man she's ever been with, something that must've been difficult for her, considering her past. Even leading up to sex, she trusts him in their day-to-day and with the massage. Now that she trusts him enough to have sex, the collar is responding to him, which could mean it's promoting a loyalty in Alison towards Torrin.
Actually, looking over this entire chapter, color plays a HUGE part in the progression of the story. It starts with Pink which is tender and compassionate, to Turquoise with control and self-sufficiency to black with fear and mystery, to indigo with intuition, to pg. 15 where Alison is literally Pink in contrast to Torrin's Blue, to sexy time in pink (back to that unconditional love), to Alison through her collar being the one who's blue.
Admin-sama, do you have a color fetish or am I just bananas??!
I definitely change to color of the scene intentionally to convey the mood, and I've tried to make different magical effects look different and stick out more than the mood lighting. The girl in Dark Altar is being directly influenced by magic, so her eyes glow. Mistress's eyes glow occasionally due to her innate magical nature.
I don't have an exact system worked out, I just try to make it look right.
Total cringe rereading my past comment, but your work is so good it's hard not to geek out when I'm not masturbating furiously!
I've been looking at your old sketches in the downtime between chapters, and I get the sense that whatever comes next will incorporate some of your earlier works, going all the way back to the idea of a collar that forces you to obey. I gotta say, while I absolutely love the story it does feel like these past chapters (while being so good I can stop rereading over and over) I do feel like it's been rushed a bit to establish the current storyline. Mistress was terrifying fantastic and Melody was encouragable, but I really do believe whatever you come up with next is gonna be the good stuff. I hope you don't mind all this, we're all just hungry for more!