What seems to be the case is that the colour depends on the type of station. It seems like on Interchange and Future interchange stations (Mt. Dennis, Caledonia, Cedarvale, Eglinton, Science Center, and Kennedy) the tiles are orange, meanwhile on non interchange stations its either gray or black. I haven't been able to pin down what determines if its gray or black, but it seems to be based off whether or not the station is under a major arterial, in which case the tile is black (so this applies to Fairbank, Forest Hill, and Leaside), and gray under local streets. The only exception to this rule seems to be Keelesdale which went for a gray despite being under Keele, but I guess since Keele is sort of weird there they thought to not classify it as a major arterial ¯\_(ツ)_/¯Just a nitpick but I think those name plates would be better if they were orange; it would really liven up the place. I don't really get what Metrolinx has against colour.