LEDs typically respond most to the same wavelength they emit, and to shorter wavelengths.
They work rather like the photo-electric effect, absorbing a photon with enough energy to create an
electron-hole pair, which then get swept apart across the electric field at the pn junction. The
energy is called the bandgap and depends on the semiconductor material.
White LEDs are different as they are actually blue LEDs with fluoresecent material in front.