L@@ks like the blue on your sign is prone to fading, kind of a factory defect,perhaps the blues were a little different pigment that made parts fade faster. The most you can do now is put car wax on it. To maybe bring back the fading.
(below is a sign that in the flange part is having the same trouble as yours)

https://www.proxibid.com/asp/LotImageVie...amp;url=166.jpg