Sefner, if you stick around, you will often see me write that a particular word or phrase is a "term of art" (see
this legal definition of the phrase). Such is the phrase, "under color of law."
Tom has a good answer to what the phrase means.
I would add that it means: The appearance of a legal right (action) by a State actor (officer; official; etc.), when no such right (action) actually exists.
It is by it's nature, a civil rights violation.