If someone reddens or their face reddens, their face turns pink or red, often because they are embarrassed or angry. He was working himself up to a fury, his face reddening. to become red, or to make something red
(especially of the face) reddened or suffused with or as if with blood from emotion or exertion; "crimson with fury"; "turned red from exertion"; "with puffy reddened eyes"; "red-faced and violent"; "flushed (or crimson) with embarrassment"
lighted with red light as if with flames; "streets ablaze with lighted Christmas trees"; "the inflamed clouds at sunset"; "reddened faces around the campfire"
The phenomenon of the trailing hemisphere of a planetary body being darker at shorter wavelengths ("redder") than the leading hemisphere This effect may be due to magnetospheric bombardment acting preferentially on the trailing hemisphere and impact gardening on the leading hemisphere Of the Galilean satellites, Europa displays this effect most prominently, and Ganymede to a lesser extent
the preferential scattering of the shorter wavelengths of light as it passes through a dust cloud, so that a large fraction of the bluer wavelengths of light are scattered away from your line of sight while a large fraction of the redder wavelengths of light make it through the dust cloud unaffected Dust clouds in space make stars behind the dust clouds appear redder than they would be if the dust was not there
Dimming of starlight by interstellar matter, which tends to scatter higher-frequency (blue) components of the radiation more efficiently than the lower-frequency (red) components