What color is the Universe? This may sound like a joke such as
What is green, has four legs and will kill you if it drops on you out of a tree?
but it isn't.
Astronomers have considered at what wavelength the Universe over all emits most strongly. Their conclusion...green. That is, if you could see the Universe from outside, its basic radiation would be a pale greenish hue.
This may not be too surprising. Recall the Sun is a very average star and it radiates most strongly at a greenish wavength. It looks "white" to us because of the way our eye works (see chapter 3). The Universe's color comes from more than just stars, but the end result is still green.
By the way, the answer to the riddle above is: a pool-table. Ok, it isn't very funny.