Blue wavelength is shorter than green, so guessing it has something to do with the plastic medium encasing the hologram degrading to the point where it absorbs the longer wavelength. Think UV protected glass, coated with a material that absorbs the shorter wave length UV light. Green is just above Cyan/Blue on the visible spectrum, so blocking green would allow the underlying blue to be visible. After that it is Violet and UV - I would be curious if some of the dark/black ones would be visible under UV light.