"The serf wore the blue coat, which was the customary garb of serving-men at that period"

Indentured servants were not allowed to buy any goods themselves and had to rely on their masters to provide them with clothing. Since blue dye, deriving from woad, was cheap, their uniforms were typically of this hue.

Flowering woad plant and woad pigment
Creative Commons AttributionFlowering woad plant and woad pigment - Credit: naturaldyer