The 'colour' of a particular beam of light is due to its specific wavelength. For example, sodium street lamps appear yellow because they emit radiation near 590nm. Visible light is electromagnetic radiation with a wavelength between 400 and 700nm. This is only a small part of the electromagnetic spectrum which spreads from gamma-rays which have small wavelengths 0.3m. As our eyes are only sensitive to the visible region of the spectrum, astronomers must use instruments to detect electromagnetic radiation of other wavelengths.
Computers then generate visible images from the information which the instruments collect.
The wavelength of a beam of light is stretched if the object emitting the electromagnetic radiation is moving away from the observer. This is called redshifting as it would cause blue coloured objects to appear red, red to become infrared, infrared to become submillimetre, etc. As the Universe is expanding, the further an object is from us, the faster it is receding and therefore the higher the redshift. Until recently the submillimetre and millimetre waveband was one of the few spectral regions not fully explored by astronomers because of the special observing conditions and instruments required. However, it is an important region of the electromagnetic spectrum as it contains information on cool objects in the Universe, particularly planets, and interstellar clouds where stars are forming.