Darker than Phthalo Blue, Prussian Blue is often found in the skies of Bobs frigid winter scenes. Prussian Blue is also used to darken the corners of an otherwise Phthalo Blue sky. This effect is used to bring the viewers attention into the center of the painting. One of fourteen warm clean colors, yielding endless numbers of natural combinations in a smooth yet firm consistency necessary to the Bob Ross Wet-on-Wet TechniqueŒs. No other paints can accomplish what these do, and every batch is individually tested for perfection.