I feel like sharing my methodology. This might not be the best technique, but it's my technique. This is what has developed over time, through advice, tutorials, and trial and error. This is my process for digital coloring. Don't worry, there's pictures.
Part one here, is just my method for color flats.
I start with an image, obviously. Usually scanned. I start around 300 DPI (that's Dots Per Inch), because that's print minimum resolution. Sometimes I work in 400 DPI, and some may work in 500. Screen resolution is 72, if that gives you a better idea.
I take my image and paste it into a new file, setting the image's layer to MULTIPLY.
I then fill the BACKGROUND layer with a neutral color. Sometimes, I go a little dark or light with it, depending on the mood I plan to evoke. Batgirl here is part of the dark Gotham family of characters, so I make it a little dark.
I select a dark gray from my pallets and use the FILL TOOL.
I then go into my pallets and get a dark gray, again. Then I go into my COLOR PICKER and shift that gray towards purple, making a desaturated purple that I'll use for Batgirl's primary suit color.
Learning your keyboard shortcuts is invaluable. They're all a little different on different programs and versions of Photoshop, so I didn't list them. Keep one hand on the keyboard so you can switch between tools and resize brushes on the fly. This will always speed up your process.
This, without stopping to take screen captures, took about 10 minutes to do. It's a simple color pallet and just a figure, so it's not complicated, but the basic principles remain the same, no matter how detailed the image is.