[doveman2]

You will only get 24Hz from 1080p. 24Hz is not a standard for DMT resolutions (computer monitor resolutons like 1366x768). See http://elinux.org/RPi_config.txt for what is possible.
DMT typically uses full range RGB, whereas CEA uses limited range. You can force it with hdmi_pixel_encoding, but no guarantee your display will support full range for CEA or limited range for DMT.
It's shouldn't make any noticable difference as long as both ends (the Pi and display) are using the same standard.
Any difference is colours may just be down to different display settings on the TV (brightness/contrast etc).

Ah OK. I thought it would switch resolution automatically to whatever was needed to match the frame rate but if not that's cool, I'll just run at 1080p so that I get 24hz.

The TV settings aren't changing at all when switching resolution but switching between CEA/limited range and DMT/full range would explain it. I used to notice a similar thing with my previous CRT TV, when switching from 1920x1080 to 1280x720 it looked much sharper and the colours/blacks were stronger. That TV only supported 1080i though, so it was interlaced when using 1920x1080 and 1280x720 was more like it's native resolution.

Changing guires requires a restart.

Hmm, just tried that (with xTV-SAF) with it set to 1080p before rebooting and it still looks the same as when it's set to 720p.

I've noticed something's up with the Calibration screen as well, as the first upper-left corner marker appears massive and then the next bottom-right corner marker appears tiny!