Oh, sure, but I was suggesting applying a palette of only four colours. Everything would then "round" to the nearest grey (or white or black), though I suppose if the colours are significantly changed in terms of "brightness" (gamma, I guess) or intensity, it may not necessarily work accurately (ie. colours that are different even in the original scheme might look like the same grey). I just thought it would be worth a try.
And you may be right, it may not totally work at all, but it would make for some neat comparison pics - actual/greyscaled/forced palette...