Brown, in physics, low-intensity light with a wavelength of about 600 nanometers in the visible spectrum. In art, brown is a color between red and yellow and has low saturation.

In terms of the visible spectrum, "brown" refers to long wavelength hues, yellow, orange, or red, in combination with low luminance or saturation.

There is no such thing as brown light. You are conflating two different meanings of the word "color". The color of a light source (e.g., a laser) is a much simpler thing to describe than the color of a surface.

The RGB color model, that generates all colors on computer and television screens, makes brown by combining red and green light at different intensities. Brown color names are often imprecise, and some shades, such as beige, can refer to lighter rather than darker shades of yellow and red.