Sun acts as a bleach as the UV radiation breaks down the pigment in what it shines on. Hence why you see "UV stable" labels on stuff like paint that's designed to be outside and resist fading.
The sky is blue because of refraction, the atmosphere acts as a prism and the light we see is the blue part split out. the colour changes due to the density with things like water vapour so some days the sky will be deep blue through to grey etc.
Did you know - no two people ever see the same rainbow, because every one you see is only visible from your position with the refraction of light, so you can't see the same thing even if you just step to the left, so the person standing next to you actually sees a different rainbow, albeit ostensibly in the same place it's actually very slightly different.
Also, the angle from your position to the top of a rainbow is alway 42', because that's how the refraction works.
True story folks.