Here is my (possibly wrong) understanding of the situation:
Full color resolution can be converted to YPbPr for output. On Original XBOX no chroma subsampling is applied to the game render before it is converted to YPbPr for output, but strangely, chroma subsampling (in addition to low pass filtering) is applied the render when it is converted to RGB for output on non-1.6 motherboard revisions. So if you are using a non-1.6 Original XBOX, you will not get full color resolution using RGB, but you will get full color resolution using YPbPr (component).
The Wii and GameCube render the games in YCbCr and apply 4:2:2 chroma subsampling to the render before it is converted to either YPbPr or RGB. Thus, the user gets a 4:2:2 chroma subsampled image in both the case of RGB and YPbPr usage.
I am very curious about the PS2. When playing games, is the render chroma subsampled before it is converted into YPbPr, or is the full color resolution converted into YPbPr?
Keep in mind that using poorly shielded component cables seems to be a huge problem with many PS2s and can make the color look different than when using well-shielded cables. That is a different phenomenon that I am not talking about here.