My absolute favorite tool for that kind of adjustment, when I'm working with photos, is "curves" under the colors menu. You basically just play with the line it gives to to adjust the value of any pixel - it's like working with saturation/contrast/value/lightness all at once, and you can do things with it that would be really complicated to do with the sliders. Also,the gray behind the line is a graph of how many pixels of each value you have.

I haven't worked with it in colors much, but you can set it to only do one color at a time, adjusting the values of red/green/blue separately, and with the same graph showing for the color values of each pixel.

Levels is similar to curves in some ways, but it comes with an eyedropper - you get to pick an area of the image that you want to become white/black/midtone for each color, and it adjusts the whole image relative to that.