Hi welcome to the board.
Ok lets see, your basically saying all DVDs play in black and white, strange.
Well the PAL ones should play in green screen unless you have performed the RGB modifcation.
The American ones WILL require a scart to play in colour, and as well as this a TV to display it, but you have both these things.
Check the scart cable. It should have all the pins in the end with no gaps, thats the sign of a true RGB cable needed for playing NTSC stuff, but I dont think this is the case as you said Region 2 also are in black and white.
It must be the mod, cause it should normally be green. So I presume you have modded it and it only effects DVD playback. Unfortunatly that will invalidate your warrentee, if you havent modded it then you need to ring the Sony customer care number it seems.
EDIT - Just reread your post it seems that your only having problems with a hand full of DVDs, if this is the case have you checked for compatability. I know there are a few that don\'t work, one being Walice and Gromet. It would be rare though that all the ones you tried didn\'t work, esp when there is only about 4 which don\'t fully work.
Sorry I couldn\'t offer any better advice except to ring the sony customer care number. Good luck.