Seems okay on my end.

@nded03 Could you post your system info in a gist?
Reproduced locally on an Ivybridge, Kabylake, RX 480 (18.2.x), and a GTX 950 (396.54). Dropping the mesa label since the graphics vendors are affected equally.
@nded03, are you testing with the lowest graphics preset in-game?
@kisak-valve yes, but I just realized that I only have the Red Alert Uprising expansion rather than the original. I downloaded the demo and was able to reproduce the issue in it. Seems to be entirely dependent on texture quality; any setting other than high will result in pink terrain.

@kisak-valve yes, but I just realized that I only have the Red Alert Uprising expansion rather than the original. I downloaded the demo and was able to reproduce the issue in it. Seems to be entirely dependent on texture quality; any setting other than high will result in pink terrain.
You don't get this error with Red Alert Uprising? https://github.com/ValveSoftware/Proton/issues/6459
Compatibility Report
System Information
I confirm:
Symptoms
When running the first misson of the tutorial ("Intro to the basics"), the screen is mostly... purple. See the attached screenshot.

This happens when the graphics presets (Settings -> Graphics) are set to "low" or "lowest". With "medium", the game looks perfectly normal.
Reproduction
Run the first mission of the tutorial with "low" or "lowest" graphics presets.