This OSHA safety sign isn’t far from our business.  You’ll often see signs like this where the red is almost gone and the other colors aren’t faded nearly as much as the red.  So, what are the reasons why red fades first?

First, lets define UV light. Ultraviolet (UV) light is an electromagnetic radiation with a wavelength from 400 nm to 100 nm (shorter than that of visible light).  Lets also assume that UV light is what causes fading. UV light has a shorter wavelength than visible light.  Light with a shorter wavelength has more energy than light with a longer wavelength.

Blue light is more toward the short wavelength end of the visible light spectrum (*see spectrum image). It is a shorter wavelength and has more energy.  Longer wavelength visible light (like red) is more toward the the infrared end of the visible spectrum and therefore has less energy.  Because of their greater energy, the shorter wavelengths of light cause more “bleaching” of the pigments in the paint and more fading. So, why would light with a very short wavelength effect RED more than blue? The main reason has to do with what light is reflected by each color and what light is absorbed. Red pigment reflects the long wavelength red light. Red pigment also absorbs more the destructive high energy short wavelength light like the blues and greens.  

WAVELENGTH

Blue graphics will reflect the destructive short wavelength light better than reds (and therefore reflect more destructive UV light) so they’ll fade slower.

I’m not an expert on how light affects pigment, but this is what I’ve compiled based on my research and discussions with others. Now you’ve got an answer when a customer asks why his red sign graphics faded quicker than blue graphics.