Hi Wendy,
Answering your question in full would take a while - but the short answer is thatyou can get an idea of how hot a star is what color it is. Basically the cooler stars are red. Hotter stars are yellow, white and then blue.
More specifically, we look at how the light is distributed in the Sun's spectra - the spectra is what you get if you break the Sun's light into its different colors. A rainbow is an example of a spectra that can happen naturally. We can tell something about the temperature of some objects by observing where the maximum of the color curve is.
Maybe the pictures here would be useful: http://docs.kde.org/development/en/kdeedu/kstars/ai-colorandtemp.html
Another thing we do, especially for very hot parts of the star, like the corona (the Sun's outer atmosphere), is to look at bright lines in the spectrum at certain colors (although some of these colors are in ranges of the ultraviolet that humans can't see without special detectors!). These lines can show us what kind of gases are in the atmosphere and also how hot they are. Figuring exactly how hot different parts of the atmosphere are can take a lot of effort - I am working on a project like that right now.
cheers,
Terry