I was trying to take pictures of people on the dance floor at a big birthday party. With a disco ball, green laser lights, and other flashing lights of various colors, how do you get the white balance correct? It doesn't seem to fit the mood to stand there while the music's pumping and photograph a gray card.
In my situation, I was trying to just get some interesting action shots with ISO cranked way up, no flash involved.
A grey card will not help you much; the coloured lights will throw off any auto white balance, and if you're adjusting manually, you'll still need to account for what coloured light was actually there, which will change from photo to photo.
You're more like to want to reproduce the colours well, which means balancing for the bulbs behind the coloured gels/filters, which often overwhelm the background light (but will depend on the venue). If the background light is particularly bright, this means you've got a bit of a problem, as it probably doesn't match the bulbs in the coloured lights, and you'll have to compromise at something that doesn't make the background balance unnatural, but still renders the colours well.
All that said, a quick and dirty method to get you in the ballpark: Take two pictures, one at tungsten (or slightly higher; say 3500K) and one at daylight. Compare them to see if one looks especially blue or especially orange, and pick the other one. If you can't decide, pick an in-between value like 4500K.
The reasoning is that the lights behind the gels/filters are likely to be very close to one of those values. With all the coloured light around, it's not going to matter as much, and if you need to do subtle tweaks in post, this gets you more than close enough.
I strongly suspect you won't need to tweak WB subtly in post, and will be more worried about bringing out the colours. Even then I wouldn't feel bound by any sort of objective accuracy; it's often the feeling of these situations that's the important thing, and they often feel exaggerated.
I would suggest shooting raw and not worrying about getting the white balance right in-camera, then fixing it in post.
It sounds like there may be a background lighting which is relatively constant plus a lot of strangely colored additional lighting on top of that. The background lighting is what you should try to balance for.
Ideas:
Most dancefloor lights are tungsten, so unless you're using flash you should be okay with that. However, I've found that in these kind of photos with colored light and all, precise white balace rarely matters, and according to Exif I've shot one entire evening on auto WB and I think colors came out fine (although some of those photos use flash).
I think the best way is to try out what setting looks best and then just use it. If you prefer tungsten and use flash, you can always gel it to have the same color (see How do I use gels to make my flash match the color of the ambient light?).
If you are shooting RAW, you can correct WB in post processing by finding a near grey/white object and set that as your whitepoint. This can be less than ideal, but in a pinch it's effective.
This will also work with jpegs, however there maybe extra noise, and added dataloss associated with this.
If you want to correct for WB while shooting, find a white object and take a picture of that and use that for WB correction.
I shoot in raw on auto-wb, and then adjust to taste in the raw processor. Raw processors usually offer some sort of auto-wb feature, but when it comes to a crazy mix of colored lights I'll usually just go with what looks good to my eye.
