QUESTION: 
Why does the shuttle glow with a purple color upon re-entry?

ANSWER from Kurt Leucht on October 22, 1997:
Well you saw it as a purple color but it is still the standard fireball
colors when it comes in.  It just seems purple as a trick of the atmosphere.
When light, lets say from the sun for this example, passes through the
atmosphere some light is lost from reflecting off the air itself, and a lot
more if its smoggy.  The color of the light also changes because the air acts
as a filter which explains why sunsets start turning red.  The light is passing
through more air at that low angle so more light is going through the color
filter. So more light with the frequency of red becomes visible because the
blue was reflected out of it.  That's why the sky seems blue and sunsets are
red.

The shuttle seemed purple becase the light of the fire surounding the shuttle
was diminished for one and then seemed more red than it actually was.  Put
the two together and whammo one dark red or purple looking NASA fireball coming
in for a landing.