I’ve got official PS3 component cables on my PS2 (PS2 set to YPbPr). Have also tested with a retrogamingcables SCART cable (PS2 set to RGB), and don’t notice any difference in color levels. This is with a fat PS2.
Just to confirm, you have the PS2 set to YPbPr, not RGB?
Is it possible that the TV is changing a viewing mode, possibly triggered by some sort of detection of the PS2 signal via the OSSC? I’ve noticed a couple times that my TV randomly disabled ‘game mode’ and change the Picture Mode, the latter of which resulted in a brighter image than I prefer.
Maybe worth investigating, at the TV.