First what are you recoloring? Sprites? Ignore Herr Shyguy's advice, because there is a quicker method for recoloring whole sprite sheets. In fact, you can recolor in a matter of seconds.
First, use the "Select by Color" tool (with the hand pointing at a color square or something like that).
Set the threshold to zero.
Click on any color you want.
Color all the selected pixels with the color you want.
Other pictures are a bit more complicated. Use the wand tool and adjust the Threshold. After that, use "Colorize" from the drop-down menu. This is Baby Luigi's advice.
But, I don't know what you mean by a "shaded effect". Tell me more in detail, perhaps?