Y Pr Pb is a very different color encoding than RGB. Even though the cables are Red Green and Blue, their signals aren't.
The green cable carries Luma and Sync
Red carries Red Compliment (R - Y)
Blue carries Blue Compliment (B - Y)
and Green value is calculated by the receiver.
See the color signal
conversion chart here.
There's a few threads like
this one on Amiga.org, as well. (Ironically, I can't find the thread inspired me to tinker a bit with drawing out a conversion circuit before I got a headache and remembered why I didn't become an EE.)
