I had the reverse problem getting colour from my PAL backups on the Jap PS2. I'll describe what I do and you can try and work out the reverse for your situation.

1 For NTSC backups, I work the AV cable through an NTSC-PAL converter
2 For PAL backups I work the AV cable directly from the PS2 to the TV

If I play NTSC backups via 2 they display black/white.
If I play PAL backups via 1 they display black/white.