Like tile, most paint will differ slightly in each lot produced. If you plan to use the same color thoughout your house, it's a good idea to purchase as much paint as you can at one time. This will avoid any deviations in the paint color and save you an extra trip to the store.
As a final check when taping drywall joints, use a side light to check for smoothness. The light can be a halogen worklight, a troublelight, a lamp without a shade, even a larger flashlight. Hold the light close to the wall next to the joints in several places to reveal high and low spots, or bumps and depressions, that otherwise would be easily missed.