In the following sentence, what does mean the bolded phrase?
The annals of sports are filled with great athletes of color.
The phrase in bold talks about athletes whose skin color is not white. Historically, in the United States of America, that primarily would be people of African descent (nowadays commonly referred to as African Americans). Let me just quote Wikipedia on this. Here's what it says about this term:
The term "person of color" (plural: people of color, persons of color; sometimes abbreviated POC) is used primarily in the United States to describe any person who is not white. The term encompasses all non-white people, emphasizing common experiences of systemic racism. The term may also be used with other collective categories of people such as "communities of color", "men of color" (MOC), and "women of color" (WOC). Person of color was originally equivalent in use to the term "colored", but usage of the appellation "colored" in the Southern United States gradually came to be restricted to "negroes".