When now Acting US Attorney General Matt Whitaker was on the board of directors of a company that acted as an agent for inventors, he sent a letter to a complainant inventor client, warning of possible legal consequences if the inventor persisted. Since Whitaker stated(correctly) in the letter that he was a former US prosecutor, could he have been guilty of a crime involving “color of authority?
'Color of authority', specifically 'color of law', refers to a person who holds government power and acts in a way that is unlawful using that governmental authority to assert (implicitly or explicitly) that the action is lawful.
There is no way that someone who identifies themselves as a former government officer can be doing that.
42 U.S.C. § 1983, originally part of the Civil Rights Act of 1871 reads in significant part:
Every person who under color of any statute, ordinance, regulation, custom, or usage, of any State or Territory or the District of Columbia, subjects, or causes to be subjected, any citizen of the United States or other person within the jurisdiction thereof to the deprivation of any rights, privileges, or immunities secured by the Constitution and laws, shall be liable to the party injured in an action at law, Suit in equity, or other proper proceeding for redress ...
In this section, and other similar passages of law "under color of law" means acting with the authority, or apparent authority, of a law, or with the authority of a governmental office or position created by law, such that the act ism or seems to be justifies by the law. An act may be "under color of law" even if it is eventually held to be unlawful, either because the official went beyond the powers properly conferred by law, or because the law itself was unconstitutional.
The Wikipedia article goes on to say:
Section 1983 of the 1871 Civil Rights Act provides a way individuals can sue to redress when their federally protected rights are violated, like the First Amendment rights and the Due Process Clause and the Equal Protection Clause of the Fourteenth Amendment. Section 1983 can be used to redress violated rights based on the federal Constitution and federal statutes, such as the prohibition of public sector employment discrimination based on race, color, national origin, sex, and religion.
But an action is only taken "under color of law" when the person taking it holds, or claims to hold, an official position authorized by the law, or acts, or claims to act, with authority granted by a law. In no way is simply stating that one is a former official acting under color of law.