The discussion focuses on the brightness comparison between red, green, and blue monochromatic light. It emphasizes that brightness perception is influenced by factors such as wavelength, convergence angle, and bandwidth. The human eye is most sensitive to green light at 555 nm, making it appear brighter than red or blue light. Additionally, the highest radiant flux is also linked to the wavelength and sensitivity of the eye. Overall, green light is perceived as the brightest due to its optimal stimulation of the eye's cones.