Could be that your standard definiton got messed around with?
Do you have a modchip?
If so try,
look in settings for:
Green Fix: change to Disable
Color Correction and Macrovision Fix: change to Disable
If the modchip has a setting for PAL or NTSC, make sure the appropriate setting is enabled
Are the games imports?
If so try a PS2 NTSC to PAL converter or the other way around... a y-fix is needed
The other very unfortunate possibility is a damaged/coroded laser.
Hope this helps,