Let's look at why it's not a noun:
* Common Nouns: These are general names for people, places, or things (e.g., dog, city, book). "Brown" doesn't name a specific person, place, or thing.
* Proper Nouns: These are specific names for people, places, or things (e.g., Fido, London, The Lord of the Rings). "Brown" is not a specific name.
Here are some examples of how "brown" is used as an adjective:
* The dog has brown fur.
* She wore a brown dress.
* The tree has brown leaves.