Gases and particles in Earth's atmosphere scatter sunlight in all directions. Blue light is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Because it reflects the Ocean.

Chemtrails

Because aliens painted it that way when they thought Hillary would win.

Sky is sad.

because angels have one of those blue toilet cleaning things in their toilet bowl, but it's almost run out so it's pale.