In my understanding, the red one is when Taeju is more aggressive. like each scents have their kwn color based on the emotional state of the person. the gray scent must be just a released to make euihyun know he’s there and make him calm down his nerves. I think the red scent means aggressiveness or more of a reaction to a lesser bond since Taeju and Euihyun hasn’t bonded emotionally during those times.

what’s the difference between the red pheromones taejoo released back then, and the grey one now ??