Have your color custom mixed. If you want to achieve a perfect match or find a truly unique color, most paint stores and home centers offer custom color mixing. This makes it possible to bring in a fabric swatch, painting, or other color reference, and have a paint color created to be a perfect match.

Determine the amount of paint you'll need for a particular job.  The first step is to add the width of all walls in the room together. Multiply this sum by the height of one wall from floor to ceiling. Take this total number and subtract the total area of your doors, windows, archways, etc. to get the exact area of wall space you will paint. A flat surface usually requires one gallon for every 400 square feet. Take into account the number of coats you will need to do the job right. Blue and yellow are hard colors to cover up with just one coat

If buying more than one gallon of a store-mixed paint, make sure all gallons are the same color. To be sure all gallons are the same color place all gallons in the same container and mix them together.

Decide on the trim color. Use the same color on the trim if you are trying to disguise ordinary or unattractive trim work, or if you are trying to create a uniform, monochromatic look. Use a lighter or darker color to accentuate trim work or to add subtle hints or bold flashes of color to your room.

Deciding on the trim color can be confusing but is really easy.  You can keep it simple and the same throughout your home. Let's assume you have different colors on the walls throughout your home, whether they are subtle in difference or dramatic. A very unifying way to treat your trim and ceiling is to paint them in the same color. For example: Your master bedroom may be painted a pale blue, your daughter's room lavender, the study has a textured neutral wallpaper, the main living areas are painted warm beige, and the kitchen has a patterned wallpaper in blues, lavenders, and whites. You would now paint all of the trim in your home, white or a custom blend of very soft beige. Though the effect is subtle, it keeps all of the rooms related to each other. Let's assume you are painting your walls a neutral or light color, such as taupe, beige, cream, ivory, or pale yellow.   For your moldings and doors, etc. have the paint store mix a blend of your color choice cut with white. A common formula for this is one part color to three parts white. Remember to always test a sample before total application. This same blend of color and white will be perfect for your ceiling, too.

Don’t forget your ceiling. Light colors are usually most pleasing for a ceiling, because ceilings are seen in shadow. If you'd like the ceiling to match the wall color, buy ceiling paint one or two shades lighter than the wall color (on its color chip). Or, dilute your wall color with white paint in a ratio of 25% color to 75% white.

Don't worry about the white ceilings. Do not lose any sleep over having pure white ceilings. If your ceilings are pure white, then consider painting all your trim pure white as well. Sometimes this is more desirable. Personal preference plays a larger role in this decision, rather than decorating right or wrong. Although, if budget and personal preference leads you to using a color on the ceiling, use the above formula as a general rule. Using dark or contrast colors can give a wonderful look, but must be carefully considered and tested.

Color can alter the proportions of a room. Light colors such as white and yellow are airy, expansive and cheerful. Use them in small, dark areas that you want to appear larger and brighter. Dark colors such as blue or brown can create a cozy, sophisticated feeling in oversized rooms. A long narrow room can be made to appear wider by painting both shorter walls a darker color than the longer walls.

Generally, strong, warm colors like reds, oranges and yellows tend to close a space. These colors are known as advancing colors because they jump out and meet the eye. Conversely, receding colors like blue, green and violet tend to make a room look larger because they “stand back” visually. However, the darkest values of the receding colors, like navy blue or hunter green, also tend to have a diminishing effect on a room’s size. Small rooms are best made to appear larger by painting them white or a light neutral color.

Color can lower or heighten the ceiling. A dark color of paint on a high ceiling will make it seem lower. A lighter color will make the ceiling seem higher. It is customary to paint ceilings white or off-white. This creates the illusion of higher ceilings and a more open space. Medium to dark colors will create the illusion of lowering the ceiling, which can create a cozy look if the ceiling is higher than usual. With normal height ceilings, medium and dark colors could close in the area too much creating a cave-like appearance.

Some paints change color as they dry. Color change upon drying is most prevalent in lower quality flat wall paints. As these paints dry they tend to "lighten" since the dry paint film will be rather thin. Premium quality flat wall and semi-gloss paints, on the other hand, will tend to darker slightly upon drying since the latex binder is milky white when wet and clear when dry.