I do not believe there is a turnkey option for low-color modes, but you could accomplish this a couple of ways off-hand.
1) A LUT filter. Create a look-up table assigning the color pulldown as you see fit. OBS at one point shipped with a default base LUT image, not sure if it still does, but Google for how to make one of these.
2) Create a custom shader filter with the obs-shaderfilter plugin to allow more real-time processing for edge cases.