checked that the lead between PC and monitor hasn't worked loose?
Does the fault stay the same, no matter what you do, and no matter how long the computer is left on? Have you checked to see that you can't eliminate the problem in the monitor's on-screen menu options?
When you say "both the sides are a different colour" what precisely do you mean - is there a coloured bar running down each side of the screen, or are the colours just different at the sides than in the middle, or what?
Assuming the connection is OK and the monitor settings are calibrated correctly the two most likely possibilities are:-
1. Faulty monitor.
2. Faulty graphics card.
and as the monitor is the newer of the two, I would tend to suspect that as the culprit.