Black's Law Dictionary, Sixth Edition, p. 266, ISBN 0-314-76271-X:

color of law:  The appearance or semblance, without the substance, of legal right.  Misuse of power, possessed by virtue of state law and made possible only because wrongdoer is clothed with authority of state, is action taken under "color of state law."  Atkins v. Lanning, D.C.Okl., 415 F.Supp. 186, 188.

When used in the context of federal civil rights statutes or criminal law, the term is synonymous with the concept of "state action" under the Fourteenth Amendment, Timson v. Weiner, D.C.Ohio, 395 F.Supp. 1344, 1347; and means pretense of law and includes actions of officers who undertake to perform their official duties, Thompson v. Baker, D.C.Ark., 133 F.Supp. 247; 42 U.S.C.A. § 1983.  See Tort (Constitutional tort).

Action taken by private individuals may be "under color of state law" for purposes of 42 U.S.C.A. § 1983 governing deprivation of civil rights when significant state involvement attaches to action.  Wagner v. Metropolitan Nashville Airport Authority, C.A.Tenn., 772 F.2d 227, 229.

Acts "under color of any law" of a State include not only acts done by State officials within the bounds or limits of their lawful authority, but also acts done without and beyond the bounds of their lawful authority; provided that, in order for unlawful acts of an official to be done "under color of any law", the unlawful acts must be done while such official is purporting or pretending to act in the performance of his official duties; that is to say, the unlawful acts must consist in an abuse or misuse of power which is possessed by the official only because he is an official; and the unlawful acts must be of such a nature or character, and be committed under such circumstances, that they would not have occurred but for the fact that the person committing them was an official then and there exercising his official powers outside the bounds of lawful authority.  42 U.S.C.A. §1983. 

[Black's Law Dictionary, Sixth Edition, p. 266, ISBN 0-314-76271-X]