I've noticed a couple of threads recently asking about white-balance settings. I realise these are likely for widefield, nightscape type images taken with wide or ultra-wide lenses and DSLRs. In these type of shots, you may have foregrounds that you need to consider for WB, not just the stars.
For interest, I thought I'd post a little about colour calibration of deep sky images - for example taken through telescopes, where you don't really need worry about foregrounds and you're purely looking at stars, nebulae, galaxies etc. The following is not scientific, I'm not an expert. It's just info I've picked up and applied to my images.
Firstly, what is white? The basis of all of our definitions of colour come from sunlight, from our nearest star. Our Sun is a G2V 
type star. So, in our astro images we'd like to see other G2V stars as white.
One method of achieving this involves looking up catalogues and finding a nearby G2V star. It may or may not be in the image you're trying to calibrate. You may have to slew around to some other part of the sky, take and image there, figure out what adjustments to make to get that G2V star white, then apply the same adjustments to your final image.
But it's not that simple. Depending on whether you shoot high in the sky or low on the horizon, the atmosphere plays a part and affects the WB. There are methods of factoring this in, but don't want to go into them here. Just giving a broad overview of considerations.
Another method being adopted by the team at PixInsight, the image processing software I'm currently using, is to colour calibrate the average of all stars in the image as white. They do this by having wavelet routines in the software that detect stars in the image so can apply rules to just stars, as opposed to the background, or nebulae etc. This method works quite well in most cases.
You can turn off the star detection part, and use a region of interest that you can specify. For example a galaxy. They're saying that the average of all stars in a galaxy are white. This also seems to work quite well if the galaxy if a large feature in the image.
Finally - use a colour calibrated monitor. Not point getting all scientific and precise about it if you're not viewing on a calibrated monitor. If you stick by the numbers in your calibration, the image will be right but you won't be able to enjoy the fruits of your labour and it won't look right to you.
