A while ago I submitted an article[1] about the YUV colour system, which takes into account human perception of brightness/luminance, and how it differs between colours. It's also an interesting colour system to work with, as the 'Y' channel just stores luminance, so discarding the other two channels gives you a greyscale image.

I recommend reading it if you haven't already.

[1] http://nreynolds.co.uk/blog/hsv-is-dead/