Here's why:
* Noun Phrase: "Pearl white" is made up of two nouns: "pearl" and "white". The combination describes a color, making it a noun phrase.
* Adjective: The noun phrase "pearl white" modifies another noun, such as "teeth" in the sentence "She has pearl white teeth." This modifying function is what makes it an adjective.
Think of it like this: "pearl white" is a color that describes the teeth. Just like you might say "white teeth," you can say "pearl white teeth."