Is there a switch on the back of the player to set it to the S-Video/Composite Video or the Component Video (what you're calling RGB) output?
I know a lot of players, especially the cheaper models don't have parallel video outs. So they have you choose between the video output via a switch on the back.
I'm sure someone makes something to convert Component video to either S or Composite. But it'd cost a few hundred $ at least to do such a thing. And a new DVD player would be a whole lot cheaper than that if you can't get your Yamaha to work...