Fading is a change in color with time. All colors fade over time to varying degrees.
The technical term for color fading is 'photodegradation.'
The UV part of sunlight has a chemical reaction with the pigments in ink, causing the colors to fade due to something called bleaching.
Neon (aka fluorescent) colors include unique pigments that 'fluoresce' - they convert parts of the invisible UV light into visible light, causing neon colors to look super bright.
These neon particles are highly susceptible to bleaching when exposed to UV-light; this is called photobleaching.
Getting more technical; some science behind fading.
Dye's in ink contain light-absorbing color bodies called chromophores. The colors we see are built with these chemical bonds and the amount of light that's absorbed in a particular wavelength.
UV rays, visible light, and solar heat break down these chemical bonds and fade the color.