Color Choice

[Alfadog]

You can do the same thing as what pressedsteel describes with an in-built function of Photoshop. It's in Image>Adjust>Replace Color. Use the eyedropper to select base colour, and the eyedropper+ to add extra shades. Doesn't allow you too much control when colours are very similar, so in that case you could use the magic wand selection tool and/or the other selection tools to give even more precision.

P.S. Adjusting the levels (PhotoShop) will also give you more flexibility, as well as the RGB and HSB controls (both under the "Adjust" menu aswell).