First, take a peak in the tuning menu, check if there is an option to switch between different colour standards,. eg. NTSC, PAL, SECAM, AUTO.
If you find it is set to "AUTO", toggle it through the other settings. One should correspond to the correct colour standard.
If you have colour on AV input from an external source, tuning may have drifted. Try retuning or tuner/I.F. may be faulty. Unfortunately, Tuner I.F. in Mitsubishi do suffer from dry soldered joints. Although, that is a straightforward repair for an engineer to correct, some will quote for new Tuner/I.F. when it's rarely absolutely necessary. In this case get a quote.
Apart from that, you'll need to have an engineer to check the video/colour processing.
Cobweb, January 2005