When using more than one can of paint for a particular job, "boxing" paint ensures color consistency.
Boxing paint is the process of combining all the paint you will be using into one large container. This is especially important when painting a large surface area, such as a home's exterior, where even a slight color variation from one can of paint to another is markedly visible.
For most interior jobs, an empty 5-gallon container works fine. To use the paint boxing technique, simply pour all cans of the same color paint into the large container, and stir to combine. Whether you are mixing 2 or 20 gallons of paint together, the key is to make sure all the paint is combined to ensure a single, uniform color. Once the paint is combined, you can pour it into a smaller can or paint tray for ease of application.