If you remove an electron from the surface of a metal, and then accelerate it through a voltage difference of 1 volt, it will gain an energy of 1 eV, (one electron volt). When that electron impacts a metal target, it may release that energy as a photon. The wavelength (colour) of that photon is inversely proportional to its energy.

X-rays have a wavelength of about 30 pm = 0.03 nm, so the required energy is 1239.84 / 0.03 = 41.3 keV, which will require a voltage of 41.3 kV. X-ray machines operate between about 20 kV and 80 kV.

To produce red light, λ = 700 nm. V = 1239.84 / 700 = 1.77 volts.
That is the voltage needed to light a red LED.