When you want to scan a black & white line drawing into Photoshop and then add color, it is really very easy. The hardest part is finding a tutorial that shows you how to do this, so I created one...
1. Here is a photo of my Mac desktop. First you must realize that your drawing will be on it's own layer, so we will begin to create those layers. Select "New Layer" from the drop down menu either on the Layer dialog box or on the top bar under "Layer".
2. The New Layer box will pop up and you will see "Mode" as an option underneath. Click "Multiply".
3. Now you are ready to "Place" your drawing file into that new layer. [Make sure that "Layer 1" is highlighted in blue (the one you just created) so you will be placing into the layer with the multiply mode. This should just happen automatically.]
4. You will see your picture file on that layer with the file name in the Layer dialog box. The background will appear transparent in the little dialog box picture. The reason it doesn't look that way on the big picture is because the original background layer is white.
5. Now we're ready to color the drawing! You need to create another new layer for this. (This time you don't want to click the multiply mode... just leave it in "normal".)
6. If you look at the Layer Dialog box, you'll see your new layer in blue. Drag that layer below the picture layer so that when you color, the color will appear behind the line art. (OR...If you do want color on top of your drawing, then leave the layer above your original picture layer.)
7. Now you can click on your Brushes and select a Color to add spot color to the drawing. I colored the book red. I will rename this layer "spot color" by clicking on the "layer 2" name and changing it. That helps keep the picture organized. I would suggest creating as many layers as you need for different colors. That way if you need to go back and change them, you can do so without affecting other color layers. I also use a Wacom Bamboo tablet because coloring with the pen and tablet is much easier than using the mouse or especially easier than coloring with a touch pad!
8. If you want to add color to the entire background, then select the whole area, choose your color, and then click "Fill" under "Edit" on the top bar. I changed the name of the layer to "background color". Don't forget that you can change the texture of the color layers and use other filters too. (If you're wondering how to make the faces light or anything else for that matter, then you'll have to do that in a separate layer on top of the background color layer.) I also want to mention that you can change the color of your lines in the original drawing layer. That's a whole other tutorial and I'm not as savvy with that yet.
9. Don't forget to save your file!!! When you are all finished, then you can go into "Flatten Image" under "Layers" and save it as a jpeg or any other format... this merges all the layers together into one and creates a much smaller file. BUT... keep a version with all the layers saved as a .psd so you can go back and change it if you need to!
I hope this easily explains how to add color layers to your drawings. Please feel free to ask me any questions. I'm not a technical person so this is written for someone like me who just wants the basic know-how and wants it simple! Have fun! ;D
For a more advanced method with much more control over fine detail on the edges of your drawing (such as with pastels), go to Lynne Chapman's post to learn how!