It's Hogwarts, so it doesn't take long to find an unused classroom to practice in. In fact, it takes about five minutes of going down the fourth-floor corridor and trying doors until Kestrel finds one with no desks in it.
What takes significantly longer is finding a block of open time, with Jay's brother and sister monopolizing (duopolizing?) his time on the weekends and with both Jay and Kestrel being in difficult classes and legitimately needing to study, but they're Ravenclaws, they can get away with skipping History of Magic and learning on their own like they always do. Besides, any reason to leave History of Magic is a good reason.
Jay doesn't know what the test is going to be before the day they test it. That's how these experiments are always run. Kestrel knows it's for a good reason, and he knows that Jay has done exactly the same thing to him before, and he knows that either one of them is absolutely allowed to bow out at any time for any reason, but he still feels bad about it.
"Are you alright? Your hands are shaking," Jay says, and Kestrel takes a moment to wonder how he knows before dismissing the question.
"Yeah," he says, "I'm fine. It's just, um," and he doesn't say what it's just.
The door to the classroom they've been using latches shut behind them. Kestrel specifically doesn't lock it.
He turns to Jay. Kestrel knows that Jay can't tell which way he's facing, but it makes him feel better. "I was looking through Charms textbooks, and there's a spell in there to turn things different colours. I thought we'd try red," he starts off.
Kestrel knows he tends to over explain things when he's nervous, but he doesn't clamp down on the impulse; Jay's mouth might not be smiling, but the rest of his face is crinkled up the way it does when he's enjoying something.
"We know, from last week's experiment, that you have to either care strongly about a spell or know what it does in order to use it correctly. Given that you clearly don't feel strongly about the colour of random objects in this room, and you don't know what the colour red looks like —"
"Yes I do," Jay says, not quite dismissive but definitely more casual than Kestrel typically expects from him, and Kestrel stops short.
Jay is blind. Jay was born blind. Kestrel knows this because, however irritated Jay gets when people try to tiptoe around the subject, he'll tell you pretty much anything if you just ask. "How?"
Jay does not, however, tell Kestrel this. After a few moments of waiting for a response and not getting one, Kestrel shakes his head to clear it and keeps going. "Um, alright, so this won't work. Is there anything else you've heard sighted people talk about but you don't know what it is?"
•••••
They wind up trying the experiment for real three days later with a spell Kestrel managed to hunt down that turns things polka-dotted, which Jay could have understood but nobody had ever explained to him, and Jay can't do it no matter how many times they go over the incantation which means that Kestrel's hypothesis has been supported if not yet confirmed and he can reach some tentative conclusions.
About the magic, anyway. Kestrel still isn't sure how Jay knows about colours, and he hasn't made any observations that would let him guess. (Normally he would hold out for a formal hypothesis, but with Jay he often has to settle for guesses. Jay is frustrating like that.)
Fortunately for Kestrel, Jay opens the subject first. "I know what shape your face is," Jay says, after a long quiet period while they're working on ancient runes homework in the library, and Kestrel knows he does (Jay's hands had been strangely cool, his thumbs ghosting over Kestrel's cheekbones, and it was more than a year ago but Kestrel isn't ever going to forget that feeling) "but I don't know what color your eyes are, and I'd like to."
Kestrel blushes enough that even on his skin you could see it. "Greenish brown," he says, as evenly as he can.
Jay nods, crisp and even. "When I dream about you they're always blue," he says, in lieu of an explanation.
Jay goes back to his paper, and doesn't mention it again.
He dreams about me. He dreams about me often enough to notice an 'always,' is all that Kestrel can think.
(Kestrel dreams about him too, that night.)