I'm not sure that you'll get a swift definitive answer here because this is fairly uncharted territory. You've downloaded someone else's rip and said that the intro works but the game is in near B&W. I'm guessing here (not having my copy of the GT3 DVD to hand being at the seaside and all that), but the intro might well be a DVD clip (yes you've put it onto CD) that is region free - then it would play in colour. The game would be NTSC and if it had been ripped properly, that is what would come out of the PS2 AV port. In all other cases I've come across where the near B&W picture appears, an NTSC-PAL convertor is a sure fix.

So which convertor did you use and with what leads (exactly)? Did you try video jacks (yellow/white)? Did you try RF?

Have you patched the other (successful) NTSC games for PAL or do they run through the NTSC-PAL convertor?

Maybe this is cheeky, but contact Chris007 on this forum; he's got his US PS2 but not GT3 for which he is gagging! Send him the CD and see what happens on his PS2 (if he's got it chipped).

Hope this helps and look forward to your answers.