I had this same problem with Verdant Citadel a few years ago, something with the color grading. I forget how I fixed it, but from skimming through the thread you can try a few things:
- updating GZDoom
- updating video drivers
- switch from Vulkan to OpenGL (or vice versa)
- disable color grading in "Display options > Hardware renderer > Postprocessing", or enter "r_colorgrading 0" in the console
also first post, hello :)