Picasso initially added color to Guernica through bits of newspaper, blue shadings and a red tear streaming from the screaming woman's eye. It was only in the final phase of composition that he removed all color from the painting.
Color theory views black and white as non-colors because adding them to primary, secondary or tertiary colors only makes those hues darker or lighter (rather than a new color).
Guernica is portrayed in monochrome---black, white and gray tones. Picasso learned from his Blue Period that color may distract from a painting's meaning, so he removed color as a way to enhance the painting's impact.
Guernica does not literally represent the event that motivated it, but rather is allegorical. Picasso believed allegory in monochrome was best suited to evoking the depths of suffering caused by the event.
Guernica was commissioned by the Spanish Republican government to decorate the Spanish Pavilion at the Paris International Exposition (the 1937 World's Fair in Paris).