hi Gavin... some great shots you have. Just tot of some areas to try out the next time you shoot:
White balance:
You can try using auto-white balance for shooting, but review the pic after each shot. If the shot is off and auto-white balance does not work, then go into manual mode. In most ballroom settings, it has very warm lighting, so if you are not using flash, you can set the white balance anywhere from 2800-3700K, depending on the situation. Got to experiment with it to see how it turns out.
Flash photography:
I would mostly use flash in such a setting because of the lighting. You can go to
www.abetterbouncecard.com to make that better bounce card and also learn how to use flash. I recently tried the instructions from this web site and made a bounce card, and it gives pretty even lighting on the faces of the subjects. When you use flash, the lighting from the flash would change the white balance as well, so try auto white balance first and see the situation. Also, note the distance your flash can throw the light, so adjust yourself by taking some test shots with a subject to estimate what would be an optimal distance.
Lens:
If you use flash photography, a normal lens (i.e. not fast lenses) are sufficient for taking the shots. 70-200 would be too much normally, so if you have a 17-55 lens would be a good range.
Different subjects with different skin tones:
I have recently realized that the white balance setting to take an indian with a darker skin tone would be slightly different from a chinese with a lighter skin tone. So this is something I am learning at this time when I take shots.
Post processing:
After taking the shots, it is important to do some post processing. There are free programs like GIMP or Picasa2 which can do quite a good job.
Cheers and happy shooting!