This is a great question. As you suggest, the color of an object can tell us something about its temperature.
It turns out that an object which is at a fixed temperature will
give off light over quite a broad range of wavelengths. The center, or
average, of these wavelengths are related to the temperature. If you
heat a piece of steel until it looks white, it is quite hot. The
wavelengths of the light that this piece of steel gives off range from
red to blue, which (when mixed together) look white to our eyes.
If something is even hotter than this (so hot that it looks blue to
our eyes) it means that the wavelengths of the light that this object
gives off probably range from blue up through the ultra-violet (UV).
Since our eyes cant see UV light, it just looks blue to us.
So, unless something else is going on, an object that looks blue
probably has a higher temperature than something that looks white.
Having said this, you should realize that this may not always be
true. Some chemicals burn with a blue color, for example, so that if
you burn some of these on an ordinary fire it will look blue for a
while (some fireplace logs may do this). This does not mean that the
temperature of the whole fire went up, just that these chemicals made
the color change.
(republished on 07/24/06)