If it is a CRT it shouldnt be permanent. I had speakers next to an old CRT and it went all crazy and colourful in the corner but it was fine when i took it away.
Its just physics; in CRTs electrons are fired from the rear of the box onto the back of the glass and the colour you see is formed by, you guessed it, magnets! So puttting a magnet next to it pulls the electrons along a warped path and gives you some weird colours. It shouldnt be permanent unless you screw up the monitors magnets at its rear.