4. Mistaking swatch for wall. Getting the "right" version of a color can be tricky. Colors will look darker on the wall, but the glossier the paint, the lighter it will look. First, use swatches to narrow your choices. Once you have a handful, get small containers of the paint, and test the finalists on big pieces of poster board, says Sharon Hanby-Robie, designer and author of "Decorating without Fear." Prop the boards up in the room you'll be painting. Look them over the next few days in all kinds of light conditions, from daylight to nighttime illumination. "That's the fun thing about color -- it changes with the light," she says. Shades can also take on aspects of the other colors near them, like carpet color, which is why you want to test it out in the room before you paint. After a couple of days, you should have a favorite. And if you don't, at least you don't have to live with a bad choice until you repaint.

5. Ignoring warm and cool. Not acknowledging that color can change the perception of a room. Warm colors "are expansive," says Gillingham-Ryan. Use them for social areas, like a great room or dining room. Cool colors are calming. They can be great for a home office or bedroom.

6. Forgetting every wall is different. Want to give a room punch by painting one accent wall? Select an unbroken wall (no doors or windows). Often it's the wall you see first coming into the room, says Gillingham-Ryan.

7. Forgetting the ceiling. "It's the fifth wall," says Hanby-Robie. But even experts disagree on how it should be painted. If you expect to paint your ceiling a color, consider going a few shades lighter because ceilings tend to look darker. Some designers prefer a white ceiling. A decorator's white (not a stark white) will give you a sharp definitive line from painted walls and "highlight whatever architecture you have," says Gillingham-Ryan. Others want the ceiling in the same color family as the walls. "If you paint the walls a rich color and leave the ceiling white, it will do the opposite of what you want" by calling attention to it, says Lowell. Instead, he prefers a variation on the wall color. If your ceilings are above nine feet, go one shade darker than the wall color, he says. That will bring the ceiling "down" and make the room appear more cozy. If the ceilings are nine feet or less, use a color a shade or two lighter than the walls.

8. Not using color to connect rooms. Especially those that flow together. One shortcut to getting a pro look: Take a swatch that you like with seven versions of the same color. Eliminate the darkest color (you can use that to accessorize the room later), says Lowell. Now go two shades lighter. "That's a great wall color," he says. Try two shades lighter than that for the ceiling. And use the lightest shade on the card for your trim. Want to coordinate two adjoining rooms? Use the same trim color, but switch the wall and ceiling colors, he says.