This shader is based on Flyguy's awesome "Apple II-Like Artifact Colors" shadertoy and is the most impressive/magical of the shaders I'm going to cover here. Where this shader excels is in reproducing the NTSC "artifact colors" that certain old computers depended on before full-color interfaces were a thing. You can find some great explanations of the phenomenon at the 8088 mph demo writeup and this post at nerdlypleasures.
This splitscreen image demonstrates just how mind-boggling this effect can be, taking a 1-bit black and white image and ending up with bright colors:
|RGB output on the left, composite artifact output on the right. Those colors are all generated by the signal modulation :O|
|Notice how the magenta and black stripes turn to a rich brown|
I also made a preset that pairs it with T. Lottes' CRT shader, which is perfectly suited to CGA content:
|I called this one c64-monitor in the 'presets' subdirectory of the shader repos|
This preset also looks pretty darn good for general use, though it does a strange green/magenta strobe thing on Sonic's waterfalls and probably other pseudo-transparency:
This one from J. Kyle Pittman / PirateHearts is similar to the CRT effect in You Have To Win the Game:
|This looks gray, but it's actually red, green and blue diagonal lines|
Both of these shaders are available in RetroArch's regular GLSL and slang/Vulkan formats. On-topic but not pictured are two NTSC shaders derived from MAME's NTSC shader implementation, one multipass and one single-pass. I haven't gotten them to work properly in GLSL format, only slang, and all of my Vulkan screenshots have their red and blue channels swapped for some reason right now, so I couldn't share any shots of them. You can get a taste from their shadertoy implementation, though.