This page lists the exposures on a set of signs dated December 2, 1987. It is interesting to note that the "via" points are grouped by corridor on this roll, whereas the 1998 sign has them listed in straight alphabetical order.
Along Yonge Street the "Yellow A" service was express, while "Blue B" was semi-express and "Red C" was local, with the bus stops colour-coded accordingly. ("Black D" was never used.) Red was chosen as the color for the local service so that it was not necessary to modify the old Toronto Transit Commission bus stop signs north of Steeles Avenue. (Green is, of course, GO Transit's corporate colour.)