Cool, but I would give the belts numbers.
Probably couldn't use numbers as these are not state highways. I don't know if PA recognizes numbered county roads.
This was brought up in another thread, but the concept of “county roads” is almost foreign in Pennsylvania. In many counties, the only county-owned roadways are essentially driveways or access roads on county-owned facilities (county parks, county-owned reservoirs, county prisons, etc.). Allegheny is one of the few counties (perhaps the only one) to have any general purpose streets/roads under county ownership and maintenance.
I think the use of colors is appropriate since the belts traverse a patchwork of state and local roads, some of which are already numbered. I have to wonder how much the colored belt system is actually being utilized, though. I remember seeing one AAA city map for Pittsburgh that had the colored belts highlighted (and explained), but I imagine that most out-of-towners have no idea what the colored belts are and don’t use them. And I assume locals wouldn’t need them for the most part.