With all Audi navigation systems in the US it has been this way:
Interstates: blue with a thin red outline
Tollways: yellow with a thin red line
Major roads: red
Minor roads: yellow
Local roads: white
Even smaller roads: grey
With TMC enabled traffic is shown for interstates, tollways and some major roads only, as additional line next to the road. The road being red or yellow has nothing to do with traffic. And even with Goggle Earth enabled, traffic is not shown on all the roads even though Goggle has traffic information for them. Traffic is Sirius based which is only available for interstates, tollways and some major roads.