It seems like rendering effects bug out mount textures? Whatever it is, it depends on the map and mount skin. I get all my issues from my Gallant Lightbearer and Open Ocean Glaucus
When I enter Iron Marches my Gallant Lightbearer tuns into this gross dark green color. https://imgur.com/a/lFF7dFE
When I enter ether Wayafarer Foothills or Harathi Hinterlands my Open Ocean Glaucus turns green. https://imgur.com/a/wR7xSbB The jewels on my Lightbearer also turn black and the shiny particles are red.
They look the same on the mount tab, and I cannot change their colors back unless I leave the map when the colors will change back to normal.
Haven't tested for all maps but this is what I've seen so far.
PC Specs:
Radeon RX 580 8GB
Intel(R) Core(TM) i5-7600 CPU @ 3.50GHz
16 GB RAM
Windows 10
My graphics settings: https://imgur.com/a/a2qAAZ3