Colons can be used to separate 2 full sentences. Unlike a semicolon, a colon can or cannot be for 2 full sentences. Colons are commonly used for lists, definitions, and explanations. These sentences are properly punctuated.

He achieved great feats: finishing college at eighteen, learning new topics, and making many friends.

The ACT is a standardized test: the test has four sections.

Do not use a colon if the sentence has a word like INCLUDING or a phrase like such as.

She purchases many items at Costco including cups, plates, and printer paper.