Not all members of the military have military ID cards. The Common Access Card, which is commonly known as a military ID, is issued to active duty uniformed service personnel, Selected Reserve, Department of Defense civilian employees and eligible contractor personnel.
There are several different type of CAC cards that the military issues. The type of CAC that a person receives is usually linked to the type of work that person does as well as that person's classification within the military. For instance, contractor personnel usually receive green CACs. This coloring makes it easier to identify these personnel as contractors.