What color on the spectrum is closest to an x-ray?

An X-ray is not a color on the visible light spectrum. It is a type of electromagnetic radiation, like gamma rays, ultraviolet radiation, and microwaves. X-rays have shorter wavelengths than ultraviolet radiation and longer wavelengths than gamma rays. The color on the visible light spectrum that is closest to an X-ray is violet, which has the shortest wavelength of all the visible light colors.