Picture tubes (or CRT's) have magnetic properties which must be properly aligned with Earth's magnetic fields. If a TV is moved when on, frequently color spots will appear and adversely affect the color and picture. 

The larger the CRT, the more sensitive it is. Some 31" and larger TV's have a N-S/E-W alignment switch. A common mistake is place speakers next to or (worse) on top of a TV. Speakers have magnets which will affect the purity. Color TV's have an automatic degaussing (demagnetizing) circuit built-in. But it produces a weak field and may take a couple of days for it to go away. If it does not, contact a service company and they can come out and manually degauss your TV. In some cases a purity alignment may be needed, especially if the TV has been subjected to a fall or has been handled roughly. This is when all three color guns of a picture tube are aligned to the 3 primary colors, Red, Green and Blue. Using a special video generator, a service technician can select and display a picture with just one gun on.

In rare cases, an internal component of the CRT called the "shadow mask" will detach and can cause this problem. Most of the times, when this occurs, the TV has been subjected to a fall or blow to the cabinet.