Like I'd be interested to see if its like a pH kind of thing, you know? but like even then man, I cant imagine it reacting so quickly
It's called YAOI LOGIC
In omegaverse people have pheromones in their bodily fluids, some stories even mention that different bodily fluids have different amounts of it too. I'd like to believe that since he was out and about he was likely sweating and he was holding onto it for a while talking to the guy so his sweat might have gotten onto it which caused it to change color. And since he releases alpha pheromones through most of his body naturally, the test was blue like in that one chapter where the brother was holding a device to measure pheromones in air while teaching Junhoo how to control his pheromones and the blue levels went up as Junhoo got excited and released a bunch of pheromones.
As a person thats currently in lab hell, don't know a single test where LITERLLY touching the test results in a reaction, theres like no like chemical reaction there, I guess other than the pheromones but he wasn't expressing any plus itwouldn't just be where it was touched so like YOU KNOW