Clipping masks are very powerful in Photoshop. They let you use the artwork on a layer to limit the visibility of linked layers above it. The color of the masking layer doesn't matter, only its opacity
What I got wrong last week: The clipping mask sits below the area you want to paint, not above it.
Steps:
Create a new layer
Create a solid shape that will be the mask
Create another layer above the mask layer
With that layer selected, choose "Create Clipping Mask" from the pop-up menu
From Adobe Help System:
A clipping mask lets you use the content of a layer to mask the layers above it. The masking is determined by the content of the bottom or base layer. The non-transparent content of the base layer clips (reveals)
the content of the layers above it in the clipping mask. All other content in the clipped layers is masked out.
UPSIDE: Even more flexible than the four techniques so far. About the same amount of prep-work as Color Behind Lines. Once created, you can paint past the edges of the area you want to paint.
Color In An Alpha Mask
Alpha Masks are also very powerful in Photoshop. You can create them with the Lasso tools, Marquee tools, Magic Wand and Quick Selection tools. You can save them in the Channels panel.
The advantage of an Alpha Mask is that only the area inside the mask will be affected by actions in Photoshop. Actions like painting with a brush, creating a gradient, applying a filter or effect, and any other action that changes the color or transparency of a pixel.
UPSIDE: Very flexible and quick to create (sometimes).
DOWNSIDE: Easy to get wrong.
Above is an example of how pixels are affected by an Alpha Mask.
Flatting: A Quick & Dirty Way
I have just shown you the hard way to color in the areas of a pen and ink or line drawing. Lots of steps and alternate ways of approaching the project. It's more work, but much more flexible than the following technique. THis video demonstrates that you can also do it quick and dirty, which—when your working on a tight deadline and/or budget—might be the right and best way to do it.
The downside is you don't have the flexibilty that the methods I demoed give you. So, here's the quick and dirty way:
Make sure you've got your lines on a layer or layers of their own.
They don't even have to be on transparent backgrounds. If they aren't, just set the Layer Blending Mode to Multiply.
Create a new blank layer behind the lines (Layer 2 in the image above). If it's a layer named Background and locked, then double-click on the lock icon to unlock it, then make a new layer and move it behind the lines layer.
Using the Lasso tool, select the area you want to color.
In the video, they suggest turning off Anti-Aliasing for the Lasso tool, which makes it easier to re-select an area. Not a bad idea, but unless you intend to re-color the area multipletimes, it's not really necessary
Use either the regular Lasso tool or the Polygonal Lasso tool, whichever you're more comfortable with. If you use the regular Lasso tool, make sure you use a Wacom. If you use the Polygonal Lasso, use the mouse (much easier and more precise than the Wacom)
Once an area is selected, fill it with the color you want with the Bucket tool (shortcut key G).
Fill in all the areas. If you fill in each of the areas with only a single flat color, it's easy to re-select the area with the Magic Wand tool and change it.
Change the color with the Bucket tool (under the Gradient tool), or...
