Rubor is inflammation due to:

Ref: Robbins Pathologic Basis of Disease, 8th editionExplanation:"Dilatation of the arterioles results is increased blood flow, which is the cause of heat and redness (erythema) at the site of inflammation" (Ref: Robbins)Celsus. a Roman writer of the first century ad, first listed the four cardinal signs of inflammation:Rubor (redness)Tumor (swelling)Cal or (heat)Dolor (pain)Vascular changes in Acute inflammationChanges in vascular flow and caliber begin early after injury.Vasodilation is one of the earliest manifestations of acute inflammation.It first involves the arterioles and then leads to opening of new capillary beds in the area,The result is increased blood flow, which is the cause of heat and redness (erythema), at the site of inflammation.Vasodilation is induced by the action of several mediators, notably histamine and nitric oxide (NO), on vascular smooth muscle.Vasodilation is quickly followed by increased permeability of the microvasculature, with the outpouring of protein-rich fluid into the extra vascular tissues.