Here is an example of a comic strip panel with the final results. In the tutorial I will be using Photoshop CS6, but the tools are available in previous versions.
Open up your black and white artwork, making sure the line art is on a separate layer than the background layer, as seen in the example. This method works best with simple comic-style artwork, but can be adapted to other styles. We’ll be adding the color to the bottom layer, so make sure it’s selected.
With the background layer selected, pick the Magic Wand tool (W). Make sure the settings are: Tolerance: 0, Anti-Alias: Unchecked, Contiguous and Sample All Layers: Checked.
Click on the area of artwork you want filled with color.
Now don’t fill the area with color yet! You will notice the marching ants are inside the boundaries of the line art. We want the color to fill a slightly larger area. To fix this, go to Select > Modify > Expand and pick somewhere between 3-5 pixels. This will ensure there’s no white left when we fill.
Fill the area with your desired foreground color using Opt + Shift + Del (or Alt + Shift + Backspace for Windows).
Continue this for each area of artwork until it is fully colored. Any areas that the Magic Wand didn’t select properly can be quickly selected and filled using the Lasso tool.
Now for the shading. Pick the Lasso Tool (L) and create a shape that will become the shadow. You want to give the object conjure and shape, so make sure the selection gives that impression, as seen below:
Pick the Paint Bucket Tool (G), making sure the Mode is set to Multiply, Opacity: 65% (or lower), Tolerance: 0, Contiguous: Checked, with Anti-Alias and All Layers: Unchecked.
Now with a foreground color of 50% gray or lighter, fill the selection with the Paint Bucket. Continue Steps 5 & 6 until your flat colors have the desired shading.
Tutorial by Adam Casalino