I believe that it is not just the luminosity that makes it tough to read red text on a blue/green background. The focal length of any lens depends on the wavelength of the light passing through it. which means while red light will be focused on the retina the blue region (higher wavelength) is out of focus. This puts strain on the eye and it becomes tough to maintain focus. (This effect is more pronounced when viewing red over blue / blue over red since they differ in wavelength). You might have observed that viewing red text over blue give some sort of funny feeling and your eyes get tired pretty quickly.
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I believe that it is not just the luminosity that makes it tough to read red text on a blue/green background. The focal length of any lens depends on the wavelength of the light passing through it. which means while red light will be focused on the retina the blue region (higher wavelength) is out of focus. This puts strain on the eye and it becomes tough to maintain focus. (This effect is more pronounced when viewing red over blue / blue over red since they differ in wavelength). You might have observed that viewing red text over blue give some sort of funny feeling and your eyes get tired pretty quickly.
More on Chromatic Aberrations here