X-rays can't be seen with the human eye, and don't have any "color." Images taken by telescopes that observe at the "invisible" wavelengths are sometimes called false color images. That's because the colors used to make them are not real but are chosen to bring out important details. The color choice is typically 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.
