You might have heard of "l'heure bleue" or the "blue hour" -- that magical time after twilight and before evening when the sky turns an indescribable rich shade of blue. (Also occurs before dawn.) This is our favorite shade of blue and very hard to recreate in paint.
The (abbreviated) scientific explanation for this spectacular color is the sun dropping below the horizon line to about -6 degrees. The ozone layer absorbs certain wavelengths of light, leaving the shorter wave blue light the most visible.
One color that evokes l'heure bleue is IKB or International Klein Blue, a color attributed to French avant-garde artist Yves Klein (1928-1962). Klein developed the formula for IKB himself and his last works use only this color.