Want to know how on the ball Doctor Who is? In astronomy, "redshift" is a term used to describe objects that are moving away from you (which you can tell because they shift to the red end of the electromagnetic spectrum, hence the name), and "blueshift" describes objects that are moving towards you, which you can tell because they shift to the blue end of the spectrum (because the universe is expanding, most things astronomers observe are redshifted, but they come across blueshifted objects every once in a while). So the Doctor wears red in episodes that take place in the future (technically moving "away" from us, the audience), and blue in episodes in the past (technically moving "towards" the audience).
