Here's a very simple way to color black and white images in photoshop.
I know there are probably a few other threads on this, but this is the method I created (Or so I think.) First, Open up the image you want to color in Photoshop, and think of the colors you want to use. Next, create a new layer, and fill it with the color you want to use. Set the blending option to Liner Burn. Then with the eraser tool, get rid of the color on the places you don't want that color. Repeat those steps until you finish what you want to color. I turned this:
1. Scan your image at 300, maybe even 400%, with the option "Lineart". This makes your drawing/whatever only black/white. Then, open up Photoshop (Or what you're using) and change the image's properties to "RGB Color" instead of "Grayscale". Now you can easily just fill in what ever color you want, because the image isn't smooth at all. Then, when you're done, resize the image to what size you want, and (in Photoshop) use "Bilinear" or "Bicubic" smoothing when resizing.
2. Scan your image, and open it up in Photoshop. Now, set the layer with your drawing in to "Multiply" (This makes all light colors semi-transparent), and create a layer under this. On this new layer, you can put in colors, and they're showing through the layer with the drawing, because the white is transparent. Easy and looks good!