In Commandos 2 we stored the huge map images in YCbCr 420 in memory, and converted to RGB during rendering via lookup tables rather than shaders (this was back in 1999, all CPU!). We compressed the range of each component below 8 bits, running a histogram so our bits represented values inside the range actually present in the original image. We got it down to 6 bits per pixel or something close to that.

Converting via lookup tables, one for each component, made it very cheap to perform palette-style tricks like tonemapping and smooth color shifts from day to night.

[Edit: it may have actually been CIE Lab, not YUV/YCbCr, because the a&b tended to have narrower ranges. It's been too long!]