David,
Yes, only EXT1 is RGB enabled. EXT2 and EXT3 are S-Video enabled though.
Even if you plug your XBox into EXT2 or 3, I would still expect to see a colour picture, but of a lower quality to that of EXT1 (this is certainly what happens if I plug my Digibox into EXT2 or EXT3).
You may be seeing a b&w picture if you have the EXT2/3 AV input set to 'S-Video' instead of 'AV'.
The Xbox support S-Video output, so you could connect it to EXT2/3 using an S-Video connector. The picture quality should be a lot better than using a non S-Video connection to EXT2/3, but not quite as good as RGB to EXT1. If you were to do this, you'd need to make sure that EXT2/3 is set to S-Video'.
Another suggestion that people have made in the past is to connect the Xbox (RGB out) to Digibox (VCR in). The XBox signal should then be passed through Digibox, and so when the Digibox is switched off, you'll see the XBox signal. I haven't tried this myself as I use the digibox VCR in for my video recorder, so I don't know if there's any drop-off in the quality of the RGB singal from the XBox.
Hope this helps,
Nigel.