Or it can just invert the L component of all colors in the HSL colorspace at the very last stage of rendering, which only requires a couple subtractions to do i

Or it can just invert the L component of all colors in the HSL colorspace at the very last stage of rendering, which only requires a couple subtractions to do in sRGB. Unlike the unfortunately common "invert directly in sRGB", this preserves the colors changing only the brightness, and honestly it's pretty good. Colorspace nerds will no doubt complain that there are better colorspaces available, but in practice, most consumer devices implement "sRGB" perceptually such that this works better than fancier methods (which only work for carefully calibrated displays in carefully calibrated rooms).