QUESTION: 
Why doesn't the Hubble send color pictures to Earth instead of
having the computers put the colors in them?

ANSWER from Howard Lanning on April 30, 1996:
While color pictures are really neat and impressive, there is often not that
much science one can do with them.  Although current detectors do have 
sensitivities over a wide range of the optical spectrum, production of
dramatic color images is not terribly useful in extracting the information 
that really interests the astronomers.  Even if it were possible to obtain
direct color images, many problems are encountered.  Very bright objects would
saturate resulting in a combination of color producing a white image, while
very faint sources, often also very small, are simply too faint or small to 
detect any color.  Long exposures at very low light levels can even result
in a distortion of the color balance (as in the case of color films used in 
the past).  In other words, what you see isn't necessarily real.  Even
color film or TVs use only 3 primary colors, red, green, and blue.  The
variation in intensity of the light effectively changes the hue, or color,
to produce what we see as a color image.

Detectors which are used on the Hubble convert light into an electronic signal
which is read out, as noted in the Live From the Hubble Space Telescope
broadcast, as varying shades of grey from white to black.  What we see then is
a variation in intensity of the object as opposed to a color.  When studying a
galaxy, for example, which is composed of very hot stars radiating energy
primarily in the ultraviolet, and at the same time also contains cool stars
which radiate at much longer wavelengths such as the visible and infrared, it
would not be possible to tell which stars are the hot ones or the cool ones in
the resultant image.  However, each of the cameras are equipped with many
filters with eliminate all but the light the astronomer is interested in
studying.  Stars and gas are made up of elements such as Hydrogen, Nitrogen,
Sulfur, etc. Filters can isolate regions of light containing the element of
interest giving the astronomer a very accurate picture of what the nebula, for
example, is made of and how it has evolved with time.  Since some elements are
lighter than others, they behave differently as in the case of explosions of
stars.  The shape and structure of the nebula, as well as its history, can be
determined by examination of the different elements.  We can even get an idea
of the temperatures involved. 

Since we know the color of a particular wavelength of light from laboratory
studies, or that which we would see through a specific filter, we can then
combine very detailed images of faint objects after the data is available on
the ground producing very dramatic 'true' color images.  We know exactly what
elements are present to create the resulting color image, something we probably
wouldn't know if we tried to take a wide-band color image to begin with.