"Color is a function of the human visual system, and is not an intrinsic property. Objects don't "have" color, they give off light that "appears" to be a color. Spectral power distributions exist in the physical world, but color exists only in the mind of the beholder. There is no physical significance in color names. It's all a matter of culture and culture depends on where you live, what language you speak, and what century it is. A given wave of light has the same frequency no matter who is viewing it, but the person perceiving the color will call it a word appropriate to their culture" From excellent article in The Physics Hypertextbook, here:
http://physics.info/color/
Example: The spectrum of light coming from a brown object like a tree or some mud will contain all the frequencies of light in the visible spectrum, with different strengths for each frequency. You can examine this with a prism. But this broad spectrum of light is indistinguishable to our eyes from light which contains only red, green, and blue light in the appropriate mixture. If you were to look at mixtures of perfecty red, green, and blue light through a prism, you would get three sharp lines. The colors of mud and trees will give a rainbow pattern which is brigher towards red end and dimmer towards the blue end.
Found a clear explaination of what is perceived by normal human vision and called "brown" and shades of "brown" in Western English speaking world 2015:
"In Optics : Brown is a composite color, made by combining red, yellow and black.In the RGB color model, which uses red, green and blue light in various combinations to make all the colors on computer and television screens, it is made by mixing red and green light.
In terms of the visible spectrum, "brown" refers to high wavelength (low frequency) hues, yellow, orange, or red, in combination with low luminance or saturation. Since brown may cover a wide range of the visible spectrum, composite adjectives are used such as red brown, yellowish brown, dark brown or light brown."
"As a color of low intensity, brown is a tertiary color: a mix of the three subtractive primary colors is brown if the cyan content is low. Brown exists as a color perception only in the presence of a brighter color contrast.Yellow, orange, red, or rose objects are still perceived as such if the general illumination level is low, despite reflecting the same amount of red or orange light as a brown object would in normal lighting conditions."
From:
https://en.wikipedia.org/wiki/Brown