Turisu said:

Personally I can't stand the THX DVD's digitally tweaked look. I'm just making the point that there was more than simple color regrading that went into making it look that way.

I don't think there was any magic involved, this fella over at horrordvds: http://www.horrordvds.com/vb3forum/showthread.php?t=44865&page=17

managed to approximate the look by simply adjust the yellow color channel.

As you can see, it can easily be accomplished without isolating specific sections of the original image if that's the look you want.