Here's how it functions:
* Adjective: Describing something that lacks brightness, sharpness, or excitement.
* "The room was painted a dull grey."
* "He gave a dull presentation."
While "dull" can sometimes be used as a noun, it's less common and usually refers to a specific thing:
* Noun: The opposite of something bright or exciting.
* "The party was a bit of a dull." (This is less formal and more conversational).
So, unless you're talking about a specific "dull" as a noun, it's generally an adjective.