The Earth's magnetic field affects all CRT's. As you change the orientation of the tube, it interacts differently with the static magnetic field of the Earth, and throws off the purity. This happens every time you move a TV, pretty much. To counteract this, ALL color monitors and television sets have built in degaussing coils. They're powered by a thermistor, and as such, fire for a short time every time the set is turned on from being cold, then go inactive. They won't fire again until the thermistor cools down again.
So, to prevent the "rainbows", turn the TV on it's side first. THEN turn it on. It may take an additional power cycle or two to get it all cleaned up from rotating it that much, and occasionally you need a standalone degaussing coil to clear it up. But, usually, if you let the thing sit for a half hour, then turn it back on, it should be fine.
The "rainbows" aren't hurting the TV in any way. You'll see the same problem if you open or close a cocktail game while it's on, or even turn many games/TV's to face a different direction while on. Letting the set degauss (or doing it manually) will fix it up.
Computer monitors have a degaussing switch or function in the control panel that allows you to fire the degauss circuit at will. Try this at work - turn a co-worker's computer monitor sideways, screwing up the colors. Then degauss it with the front panel control, which will fix it. Then rotate it back upright into it's normal position - which screws up the colors again. Walk away.
-Ian