Painting a room two different colours brings an element of whimsy or polish, depending upon the colours you choose. Sometimes a room has a chair rail, which divides the wall in half for you. If your room doesn't have a chair rail, or if you're painting vertical stripes, you can still paint it two different colours if you do a little extra measuring first.
Determine where you want the colours to meet. Measure from the floor up to this point using a tape measure, and mark it with a small pencil line. Mark this point on all four walls every 30 cm (1 foot) or so.
Tape a horizontal line on the walls using making tape. Use your marks as a guide and a level to make sure the tape goes on straight.
Paint the first colour on the top half of the wall, stopping when you reach the masking tape. Let this colour dry completely and then remove the tape.
Reapply masking tape along the edge of the first colour you painted. Now paint the bottom half of the wall using your second colour, stopping when you get to the tape.
Wait until the paint dries before removing the masking tape. If you want to cover the dividing line between the two colours, apply a length of wallpaper border to cover the meeting point of the two colours. Otherwise, leave it exposed and show off your straight lines.
To paint vertical stripes, follow these guidelines, but apply the masking tape along carefully-measured, evenly-spaced vertical pencil lines. Paint one stripe at a time, letting each colour thoroughly dry before removing the tape and painting the next colour.