Here's why:
* Adjectives describe nouns. You would say "a brown dog" or "a brown shirt," where "brown" is describing the color of the dog or shirt.
* Nouns are things or people. "Brown" doesn't refer to a thing or a person.
However, there are some rare cases where "brown" could function as a noun, such as:
* A shade of brown. For example, "The artist used a beautiful brown in their painting." Here, "brown" is referring to a specific color.
* As part of a proper noun. "Brown" could be someone's last name.
But generally speaking, "brown" is an adjective that describes color.