My wild guess would be "authority", as in the use of "colors" to mean "flag"?
A plausible but in reality false plea intended to make the point to be decided appear to be one of law and not of fact and hence a matter for the decision of the judge rather than the jury. Also in extended use. Now hist.
1531 St. German's Secunde Dyaloge Doctour & Student (new ed.) liv. f. cxlvi, The playntyfe claymynge in by a colour of a dede of feoffemente.
1579 W. Fulke Heskins Parl. Repealed in D. Heskins Ouerthrowne 89 Hee wil giue colour to the plaintife, and apply the reason vsed agaynste priuate masse by the proclamer.
1607 J. Cowell Interpreter sig. Q1v/2, Colour (Color) signifieth in the common law, a probable plee, but in truth false, and hath this end, to draw the triall of the cause from the Iury to the Iudges.
1768 W. Blackstone Comm. Laws Eng. III. 309 An appearance or colour of title, bad indeed in point of law, but of which the jury are not competent judges.
1806 E. Lawes Elem. Treat. Pleading in Civil Actions vi. 129 It is observable that in all cases where the defence consists of matter of law, the plaintiff may be said to have an implied colour of action.
1847 Rep. Cases Court Exchequer Pleas 302/1 in Law Jrnl. Rep. 25, It‥gives express colour to the plaintiff, and states his possession at the time of the trespasses under a charter of demise without livery.
1959 Earl Jowitt & C. Walsh Dict. Eng. Law I. 410/1 Express colour was abolished by the Common Law Procedure Act, 1852, s. 64: implied colour, which had been known to the law for over five hundred years, disappeared when the Judicature Act, 1873, came into force.