A quick palette editing question

As I play with palettes, I notice that it seems completely impossible to make a color that has exactly the same RGB values (like 43 43 43) with the exception of pure black (0 0 0) which is easy to make. Is there any way to tell ZQuest exactly what values I want without having to deal with that slider? Failing at that, is there a trick to getting values like this? A lot of colors like that are a part of NES palettes so it's a pretty serious problem if it's impossible.