I am still learning how to use my PS 7. However, here is a link that I have found to be most helpful.
Most of the time I just select a large brush with 100% in the opacity and flow. A couple of swipes and the color is changed. It will only paint inside the selection so it is about the fastest way once you have sampled or selected a color.
Once you have the color selected you want to replace you have to move the sliders to achive the color you want to replace it with. The part of the image you want to replace the color on should be selected or it will change them for the entire image.
Best of luck,