There are basically two different ways you can color sketches ...correction: there are probably a *million* ways, but explained in this tutorial are the 2 basic ways *I* choose to use. Depending on the look I want I either scan the original as line art (black and white drawing) or grayscale (black and white photo) at about 150 to 300 dpi. The smaller your end product will be the closer to 150 dpi you'll want to go.
Line art is what you use when you intentionally want to eliminate the gray tones in your art, while grayscale is a good choice for scanning pencil sketches, which leaves in all those soft, varying subtleties of your pencil lines.
Keep in mind that even when you use the line art method your drawing will "soften" a little the moment you resize it, but being Photoshop allows you to keep your outline separate on it's own layer, you can always give it a quick "sharpening", just before saving it for the web, without disturbing your other layers.
As a tutorial sample I've done a quick sketch of my old landlady (uh, sorry about that) ...maybe not the BEST sample I could give, but - oh well. Anyway, this is a detail of how both methods scan in. Notice the photo setting also brings in the middle gray and texture of your paper. Again, depending on the photo-realistic look you're going or not going for, you may want to suppress that effect by adjusting your "levels" once in Photoshop - or use the line-art setting to begin with.
What scan setting you choose will determine which method of coloring you'll use....