Just to clear up the most common things:
No. This is not a broken colour mod post porcess.
Yes, I have tried resetting my video settings entirely by changing the DirectX level Garry’s Mod runs at.
Yes, I have tried verifying my game files.
Yes, I have tried uninstalling the addon.
No, I have not tried reinstalling Garry’s Mod; this will only be my absolute last resort option.
Yes, I have tried contacting the map author. He has yet to respond… To both me and several other people who ask for help with other maps he has made.
This is the map in question that caused the issue:
(I do not recommend loading the map unless you know a way to fix the issue unless you want your game to become permanently oversatured in colour)
After loading into the Geonosis map linked above, You might notice the map seems a bit more orange than originally entailed, for some reason this map makes the game start rendering about 40% more saturated than originally intended. Switching maps, restarting the game, or all other options listed above have not worked thus far to fix the issue.
Example pictures of what the problem does:
What flatgrass would normally look like based on main menu wallpapers:
What it looks like now:
My colour mod settings, for the skeptical:
To get the colours back to an even slightly normal setting I have to set my colour multiplier to 0.6:
Another example, showing what the lobby of Mobenix looks like now:
Both me, and several of my friends who were loaded into the map now have this issue, and none of us have been able to figure out what causes this. I’m not a mapper but I decompiled the map and couldn’t find what could be causing the issue.