On these buses, the Line colour is displayed by a small Chevron on the top of the bus. That will change based on line colour, So they will remain with the Azure livery, but have the chevron change based on what you want. There are currently a series of full line colour buses and trains in development so you will be able to completely customise your bus apperance

I've got a few of these buses, all of which say they have "Line color compatibility" yet they're all the same light blue color, no matter the line color. Am I missing something, or does this mean something else?

Yep, at some point in the nearish future

Will there be Props for new bus models and trams? Thank you.