Painting walls in dark colors takes a little more preparation and effort than painting in lighter colors if you want to achieve a deep-color look. When choosing a dark paint color, take the size of the room and the relative amount of sunlight that enters the room into consideration. Dark colors can make a room look smaller and become virtual "black holes" in rooms that are starved for sunlight.
Prepare walls. Pull any nails or picture hangers, spackle the holes and let dry. Carefully sand the spackle smooth. Spackle nail holes in very small areas especially if the walls are textured. Any sanded repair areas can show in dark paint jobs. You can also purchase paint texture products with spray-on applicators to try to match wall textures.
Remove any moldings and trims from the walls. If this isn't possible, apply a bleed-proof painters masking tape like Frog Tape, to the trim. If you don't have ceiling trims, run a line of Frog Tape along the ceiling where it intersects with the wall.
Prime walls with a primer that is deeply tinted with your dark wall color. This is especially important if you are painting over white or light walls. You may be able to shave a third coat of paint from the project if you use a tinted primer.
Mix the paint into a five-gallon bucket if you expect to use more than one gallon of paint to ensure consistent paint color that won't reflect slight changes in tone from gallon to gallon.
Use a sash brush to cut in around the perimeter of the walls, at the corners and along the walls at ceilings. Work quickly. As soon as you have finished cutting in one wall, paint the rest of the wall. Paint wet edge into wet edge to avoid creating stripes of paint around the edges of the wall.
Paint in a "W" pattern, using a roller applicator with the correct nap of roller cover (smooth, semi-smooth, or rough). Screw the roller assembly onto an extension pole to make quick work of the project and avoid having to either bend over or climb a ladder to paint with the roller.
Cross over your W pattern many times, moving from side to side, until the wall is covered evenly with paint. Get enough and keep enough paint on the roller so that you cannot hear it "sizzling" or "crackling" as you apply the roller to the wall. If you can hear this sound, it means you don't have enough paint on your roller and you may actually be pulling paint off of the wall.
Paint each wall and let the first coat dry for a minimum of six hours. Cover your paint bucket or cans with lids. Insert your roller head and brushes into the roller tray so they are immersed in the paint. Slip a plastic drawstring garbage bag over the tray and draw the string closed. This keeps your paint, brush, and roller wet as long as overnight.
Paint a second coat in identical fashion to the first coat. Let dry overnight. Store your brushes and the roller in the plastic bag again.
Paint a third coat if your paint color is a dark red or if the dark color stills looks a little "thin" or not as bold as you expected it to be. Some colors, especially red, require at least three coats, with or without use of a primer. Let dry overnight. Store your brushes in the bag again.
Pull up the maskings along the baseboards and any ceiling lines. Do any touch ups necessary. Reinstall any trim boards you may have removed
Store your leftover paint in sealed containers. If you have less than a half-gallon of paint left, pour it into a jelly jar and seal it with a lid. Mark your paint reserve cans with the date of your project, the paint manufacturer, and the name of the paint. If you ever need more of this color it will be easier to obtain a match with this information.