Yeah, my friend had that happen with her mailbox. It was this dark blue for years, then one morning it looked like a washed-out denim color. She even checked for paint drips on the ground, nothing. It's like @tyler492 said, things just lose their color when we're not looking. She started noticing her garden gnome was fading too, the red on his hat looked pink. Really makes you question your own eyes sometimes.
Oh wait, are you saying the sun actually could be getting stronger? I always thought fading was just cheap materials or something, but now you've got me rethinking everything.