That gorgeous blue at before dawn and just after twilight….see below for Carol Heath Stabile’s chalk pastel class and Gary Griswold’s watercolor class. If all you do is scribble in this color of blue you’ll walk away happy. Here’s the official definition obviously NOT written by an artist:
According to the U.S. Naval Observatory, civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished; at the beginning of morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visible under good atmospheric conditions in the absence of moonlight or other illumination.
Port Townsend School of Maritime Art 
Polar opposites as definitions go, but I live with plenty of that right at home: being an artist myself living with a technologist! Delightful post: now I know what to call that mysterious aura!
I am now much more interested in knowing about “uncivil twilight” and will plan a workshop to explore that concept…