When you move from room to room, consider your color story, or how the colors will interact. Using complementary colors - colors that sit opposite one another on the color wheel, will create a very dramatic look in your home. Choosing analogous colors, which sit beside one another on the color wheel, creates a more subtle appearance. Monochromatic looks can also be very effective in small spaces, when you choose varying shades of one color to give variety, but without definition.
Here is where this affects you as a consumer. You select a painter with a contract that says 2 coats, $500 down. You give the company the deposit and pick your colors a couple of days before the project starts. The painter goes to the store with your colors and figures out they are deep base. He (or she) not only needs to charge you more for the paint, but he also needs to charge you for a dark gray primer coat. Ninety nine percent of the time that primer coat is going to be really, really expensive since you already gave a deposit.