why is the setting sun much redder than at noon?

This link should answer you question exactly.

http://en.wikipedia.org/wiki/Rayleigh_scattering

aswell this link for more explanation of the colors of the sky at different times.


http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

We see colors because letina captures certain frequency of waves. We
recognize different colors because each color has its unique frequency
level.

The "light" we see daily changes its velocity, thus its wavelength as
it refracts (the ray of "light" is redirected). (In physics, speed of
light is defined by the product of frequency and wavelength.
Refraction does not alter frequency.) When it passes through
atmosphere, however, water vapor refracts the "light". Since the
degree of refraction (physicist call it refractive index) through
water (or water vapor, chemically they are identical) is a certain
constant, the ray of "light" is always refracted (in another words,
the velocity of ray changes) the same amount. Water vapor's refractive
index happen to change the speed of "light" such that new velocity of
"light" carries the frequency equivalent to bluish color.