Black & White Screen ! Please Help Me !

[Urso]

I have some PS2 back ups that work perfectly on a TV screen (color images). And I have an old monitor (of the Commodore Amiga), where I can connect the PS2, with the normal cable that came with it ! But the problem is... If I put a DVD, a PSX (back up) or an original PS2 game, I see color images (on that monitor), but if I use a PS2 back up, I see the image but only in grey scale (black & white) !
Does anybody knows the reason of it and can help me !
I would like to use the monitor instead of a TV set.
Thanks !