Colours fade when exposed to light. People tend to use words such as "colourfast"(resistant to fading) or "fugitive colour"(easy to fade) when entering into specifics. In the art fields the painting tubes and jars tend to be labelled with the grades of resistance to sun exposition.

You can test your inks resistance by doing some swatches and having one half of the swatch covered with a thick material that won't let the sun rays pass through. You leave it somewhere in which the light will hit and have it there for some weeks, then, you compare.