Capitalize "Colonists" when:
* It refers to a specific group of people with a shared identity: For example, "The American colonists fought for independence from Great Britain." Here, "Colonists" refers to a specific group of people with a shared history and purpose.
* It's part of a proper noun: For example, "The Society of Colonial Dames" or "The Colonial Williamsburg Foundation."
* It's used as an adjective: For example, "The colonists' desire for self-government."
Do not capitalize "colonists" when:
* It's a general term referring to people who live in a colony: For example, "Many colonists came to America seeking a better life."
* It's used in a descriptive way, not as a proper noun: For example, "The colonists were often forced to live in harsh conditions."
In doubt?
If you're unsure, it's always best to err on the side of caution and capitalize. But, if you're writing for a specific publication or academic institution, consult their style guide for their preferred capitalization rules.