This is the good example where one can need image processing. Usually astronomical images are taken in black & white (Grey scale images). Later we have to process those images by filtering and adding colors.All these things are very interesting. This post is taken from hubbles official site. For more information you can visit the website.
Images of astronomical objects are usually taken with electronic detectors such as a CCD (Charge Coupled Device). Similar detectors are found in normal digital cameras. Telescope images are nearly always grey scale, but nevertheless contain some colour information. An astronomical image may be taken through a colour filter. Different detectors and telescopes also usually have different sensitivities to different colours (wavelengths).
Taking color pictures with the Hubble Space Telescope is much more complex than taking color pictures with a traditional camera. For one thing, Hubble doesn't use color film — in fact, it doesn't use film at all. Rather, its cameras record light from the universe with special electronic detectors. These detectors produce images of the cosmos not in color, but in shades of black and white.
Finished color images are actually combination of two or more black-and-white exposures to which color has been added during image processing.
The colors in Hubble images, which are assigned for various reasons, aren't always what we'd see if we were able to visit the imaged objects in a spacecraft. We often use color as a tool, whether it is to enhance an object's detail or to visualize what ordinarily could never be seen by the human eye.
(continued in the official website)
Further read:
http://www.spacetelescope.org/projects/fits_liberator/improc/
