Whether its photoshop, paintshop, or any other digital medium, most have several tabs on the left nav bar (or where ever you've placed them). There are different and varying ways to color in any and all of these mediums, depending on your style, technique, and what exactly you're trying to achieve with the coloring process.
Generally speaking, theres a tool called spraypaint, or something similar. This is probably the most generally used tool for coloring large bits at a time, in any number of sizes. You could also use the pencil or "draw" tool, and achieve the same results depending on line width and density, etc.
Its really up to you in how this is achieved, but these are the two basic ways of going about coloring.