X-Ray Images & False Color
The colors we see in the world around us are the result of the way that the human eye and brain perceive different wavelengths of light in the visible part of the electromagnetic spectrum. X-rays, and other wavelengths such as radio, infrared, ultraviolet and gamma-rays, cannot be seen with the human eye, and thus do not have any 'color.' To see the invisible wavelengths, detectors sensitive to those other wavelengths are needed.
Images taken by telescopes that observe the 'invisible' wavelengths are sometimes called 'false color images.' That is because the colors used to make them are not 'real' but are chosen to bring out important details. The color choice is usually a matter of personal taste, and is used as a type of code in which the colors can be associated with the intensity or brightness of the radiation from different regions of the image, or with the energy of the emission.
Chandra X-Ray Observatory Center