Traditionally, blue has been considered a primary color in painting, with the secondary color orange as its complement.
Blue pigments include azurite, ultramarine, cerulean blue, cobalt blue, and Prussian blue (milori blue).
Scientific natural standards for blue
Emission spectrum of Cu2+
Electronic spectrum of aqua-ions Cu(H2O)52+
Blue is the color claimed by the street gangs Crips and Surenos