What's the white balance to take pictures at night of the pool with yellow light?

I have a Nikon D5100

Either shoot in RAW and edit it later or find out the lightbulb type or you can always edit the white balance on a JPEG as well.

If need be just go through each white balance option until you find the one that works best.

I'd try a custom. Look it up in the manual. Tip: set lens to manual focus when you do the custom wb shot, then switch back to AF. A dedicated WB device is best of even a sheet of plain white paper will already make a huge difference.

Custom White Balance, taking a reading is most accurate, although the meter can be fooled by fluorescent lighting strobing at 50/60 cycles per second and mixing with ambient lighting… You don't actually need white paper, taking a shot of the light source filling the frame works as well (you don't need focus for this, so switching to manual focus is easier) Take several readings until you get natural colours. By yellow light, if you mean tungsten or incandescent light bulbs, then you can choose that preset.

Indeed Nathan has summed it up well. But for a start try Tungsten, that will somewhat reduce the excessive yellow. But, hey, digital costs you nothing to shoot - experiment! There's no 'right' way, only the way that gets you what you want.

Shoot RAW - then adjust the white balance when you process the image.

You probably won't be able to use one of the presets for something like that, you'd have to do a custom white balance if you are shooting jpeg. But to be honest, it's not worth the hassle. The RAW work flow is the way to go!

Depends on the ambient light being provided and what colour you want the pool of light to become…