Though both classified as primary colours, and share many other commonalities, red and green are entirely separate entities when put in the context of the physiological response of the eye to light.
Humans and other species with three types of color receptors are known as trichromats. These species respond to the light stimulus via a three-dimensional sensation, which can generally be modeled as a mixture of three primary colors. Protanopes and deuteranopes see no perceptible difference between red, orange, yellow, and green. All these colors that seem so different to the normal viewer appear to be the same color for this two percent of the population.
The colour green enjoys its highest intensity at a wavelength of 546.1(nm) while red is most intense at roughly 700nm (Though no real display device uses such primaries, as the extreme wavelengths used for violet and red result in a very low luminous efficiency.) Additive mixing of red and green light produces shades of yellow, orange, or brown.