Here's why:
* Common nouns refer to general categories of people, places, things, or ideas.
* Proper nouns are specific names for individual people, places, or things.
"Turquoise" refers to a color, and it's a general category, not a specific name.
Here are some examples:
* Common noun: Color (general category)
* Proper noun: Blue (specific color)
Let me know if you have any other grammar questions!