The picture tube on a TV can be magnitized by the Earth's magnetic field. The noise heard when you turn a TV on (usually followed by a static noise on the face on the TV) is called degauss. What this does it remove any magnetic variations on the tube due to things like the magnetic field and speakers. However, changes to the TV in orientation to the Earths magnetic field will cause the tube to have a magnetic bias. This is most prominent when turned 90 degrees. The easy way to clear this is to turn the TV off for a little while and then turn it back on. But remember that any movement once it is on can cause the discolorations to reappear. My suggestion would be to turn the tv after you've moved it to the location you'd like to view it.