Measure the size of your TV with a measuring tape. Jot down the length and width after adding 6 to 18 inches to the sums, depending on how large you want the color block.
Tape painter's tape to the wall behind the TV in a square or rectangle shape -- depending on your TV shape. Do this in the dimensions you wrote down.
Use a carpenter's level or a measuring tape to ensure the taped area is level and centered behind the wall where the TV sits.
Pour about half the can of paint into a painter's tray.
Fill in the space inside the painter's tape with the wall paint. Use a paintbrush or a paint roller if the area is quite large. A semi-gloss or satin paint is useful to focus everyone's attention to the area and make the shape appear to pop. The space may require an additional coat to achieve the look you desire. Allow the first coat to dry thoroughly before adding another.
Wait for the color block to dry before removing the tape. This creates a frame of color behind the TV, thus adding a decorative accent to the wall.