“Color” is often used in legal contexts with the meaning of “appearance” or “semblance” and the expression “under color of…” suggests something done “under the pretext (or) guise of” something else. Thus, something done “under color of law” is done with the apparent authority of the law. Acting “under color of office” may describe someone pretending to be a public official, or a public official who abuses his position for gain. And possession “under color of title” may refer to an occupant who holds himself out as the owner of a given property, acting as if he is actually the titleholder.

Many actual colors are used in legal language in many areas of law. Here are some of the most common (there may be many more!):

RED

• red herring prospectus—a preliminary prospectus for the sale of securities that has not yet been approved by the Securities and Exchange Commission

BLUE

• blue law—a statute regulating or prohibiting commercial activity on Sundays
• blue sky law—a statute establishing standards for offering and selling securities to protect citizens from fraud
• blue pencilling—deleting or censoring the contents of a book or other work
• bluewater seaman—an able-bodied seaman

GREEN

• greenbacks—dollars; legal-tender notes in the US
• greencard—US residency card; in the EU, an international auto insurance document
• green-card marriage—marriage of convenience in order to obtain permanent US residency
• greenfield site—undeveloped land, presumably uncontaminated
• green paper—a government document that invites discussion concerning approaches to an issue of public interest

WHITE

• Whiteacre—a fictitious tract of land used in legal discourse (especially law school hypotheticals) to discuss real-property issues; when the discussion involves two tracts of land, they are generally termed “Blackacre” and “Whiteacre”
• white paper—a government report on a given subject; a detailed or authoritative report
• white-collar crime—crime committed by business professionals, usually involving a form of financial theft or fraud (may include bribery, embezzlement, insider trading, etc.)
• white knight—a party that rescues the target of an unfriendly or hostile takeover bid
• white slavery—forced prostitution

BLACK

• Blackacre—a fictitious tract of land used in legal discourse (especially law-school hypotheticals) to discuss real-property issues; when the discussion involves two tracts of land, they are generally termed “Blackacre” and “Whiteacre”
• black economy—illicit economic activity done in violation of official regulations (also called “shadow economy”)
• black market—illicit trade in goods or commodities in violation of official regulations; a place where illicit trade is conducted
• black-leg labor—a chiefly British term for “scab” (a strikebreaker or, generally, someone acting against union policies)
• blackletter law (also: “hornbook law”)–fundamental and well-settled legal principles
• blackmail—extortion or coercion of money or other benefit in exchange for refraining from publicizing damaging information about the victim

BROWN

• brownfield site—a tract of land that has been developed for industrial purposes, but is underused or abandoned due to environmental pollution

PINK

• pink slip—a slang term for a notice of termination of employment
• pink sheet—daily publication of a listing of bid and ask prices for over-the-counter stocks, published by the National Quotation Bureau

YELLOW

• yellow-dog contract—a contract of employment prohibiting union membership (generally illegal under US state and federal law)

(Sources: Black’s Law Dictionary, Merriam-Webster Dictionary of Law, Oxford Dictionary of Law, Investopedia)