Does PETG fade in the sun?
tl dr: For the most part, yes it should be color fast in the sun. It should be good for outdoor use.

This website claims the following:


  Filament materials ... for outdoor use include ASA and PETG that are perfect for use in extreme conditions without changing shape or appearance. Printed pieces do not change colour either with the sun's UV rays and do not take on a yellowish appearance as does ABS, for example.


Please realize, this is partially an advertisement (and no, I have no affiliation). What it boils down to is, PETG and ASA are going to be better for outdoor use, but every brand of each is going to be a little different in how well it reacts to the elements. Without advertising on the site, you'd need to do a little investigating on your own, with the possibility of testing different brands to come up with what you believe is the best. Beyond that, it's just an opinion.

I doubt any filament, no matter what it is made of, is going to be perfectly colorfast in the sun. The sun is a harsh beast. It will bleach a printed object over time. Some colors are going to be more susceptible than others, especially red. This is due to the color of the sun. Again, some filaments are going to be better than others, but I'd not expect any filament to be perfect.