I just want to highlight how much I don't like the implementation of the PBR terrain materials. The fact that you need the same base color material for all the materials and needs to define the whole map makes iterative workflow almost impossible and if you try to just create materials that you could paint with then you are only able to paint individual faces of the terrain and there is no feathering or fade to the brushes makes that workflow basically impossible to create anything good looking. for example I put tarmac down where I place a road and its got blocky right angled patches all along it I would like to be able to design my road network in editor not have it all planned out beforehand and have to create the groud colours on one material all combined together. It's been a while since I made a map for beam and I swear the terrain painting used to be a lot better.
You can still use different base colours for different terrains, it just won't look good. I think the pbr implementation is as good as it could be. There are some workarounds for the issue you have. You could paint everywhere you want asphalt (and therefore want to be grey in the basemap). Then export the terrain, including alphas. Then load the basemap texture and the asphalt alpha in Photoshop, and use the alpha to colour the asphalt areas grey. Blend it in a bit and bosh, all done.
