Go to the lanes tab, there you can change material per lane. If you assign the material manually then it'll change when the road updates.
I did exactly that in the picture above. However it did not change anything. Updating the road provides no effect.
This is probably due to light as it is entirely black in the picture. If you rotate you main light source a bit then it should work. This was done in 2020.1.14f1:
