We can see on a scope here the graphic display of these outputs, first is the Y or Luminance output of the YPrPb out of my Dish 211 HD sat receiver, at 0.20 volts per vertical division; it strongly resembles a Sync-on-Green output from a computer workstation and most displays that can display sync-on-green would probably lock to the sync and show an all-green screen.
The Pr signal is chrominance for red and would not show up fed to the red port of an RGB display, the reference porch voltages are not available that the display would need, nor is it a red analog representation; the signal varies to both sides of ground:
This screen below is analog red above, and sync shown below, from my HTPC after going thru an Extron 202xi, the green and blue channels would scope very much alike as all are analog waveforms of similar nature. The H and V sync channels are composited to one combined sync channel. The vertical amplitude is less to show both signals at once.
All scope screens are timed for two H scan lines; 33khz for the Dish and 54khz for the HTPC.