I ran into this expression when reading Jon Meacham's Art of Power,
He(Jefferson) was usually a master of his emotions. "I know of no gentleman better qualified to pass over the disagreeables of life than Mr. Jefferson, as he makes his calculations for a certain quality of imposition which must be admitted in his intercourse with the world," said a friend of Jefferson's. "When it shows itself in high colors, he has only to count ten and he is prepared for the subject."
I have checked the word "color" in Merriam-Webster, it contains the meaning of "nature, character" when used as plurals. But I feel it's not quite right and I don't know which definition is better fitting here in this context.
Best Answer
'in high colors' in the 1700s and early 1800s
I looked at a number of instances of "in high colors [or colours]" from the 1700s and early 1800s—encompassing the period when William Smith, John Adams's secretary, made the comment about Jefferson that the poster cites—and in all of them the expression appears to be not idiomatic, but literally or figuratively descriptive. In effect, "in high colors" means "in bright colors" or "vividly."
Here are some early instances of its use.
From a letter to Lord Hardwicke, dated September 5, 1758, in John Nichols, Illustrations of the Literary History of the Eighteenth Century, volume 1 (1817):
From a review of Cornelius Cardew, "A Sermon, Preached at Truro, before a Provincial Grand Lodge of Free and Accepted Masons," in The London Review (October 1779):
From "Monthly Register: House of Commons, Tuesday, Feb. 16," in The Literary Magazine, and British Review (March 1790, page 222):
From John M'Donald, Letters Addressed "To the Friends of Religion:" Criminating the Presbytery of Albany, and the Synod of New-York and New-Jersey, with Answers, by Jonas Coe (1801):
From the entry for Vera Cruz [Mexico] in Antonio de Alcedo & George Alexander Thompson, The Geographical and Historical Dictionary of America and the West, volume 5 (1815):
From Henry Crowe, Zoophilos, or, Considerations on the Moral Treatment of Inferior Animals, second edition (1820):
From William Webb, an entry dated July 23, 1823, in Minutes of Remarks on Subjects Picturesque, Moral, and Miscellaneous (1827):
From Sir George Head, Forest Scenes and Incidents, in the Wilds of North America (1829):
'in high colors' as used in the quotation about Thomas Jefferson
Merrill Peterson, Thomas Jefferson and the New Nation: A Biography (1970) discusses William Smith's description of Jefferson in the context of Jefferson's role in 1789 as U.S. minister to France:
As Merrill Peterson indicates, the thing showing itself in high colors was "imposition"—which in the 1700s could mean, among other things, the state of being intruded upon, or inconvenienced, or compelled to perform or accept some obligation, or lied to. Samuel Johnson, A Dictionary of the English Language (1756) lays out the possibilities in his entry for imposition:
It seems clear, then, that Smith uses "in high colors" to describe an imposition of a particularly obvious, inescapable, and disagreeable sort that may unexpectedly "show itself"—and to express admiration at Jefferson's ability to react coolly, practically, and without irritation to such inevitable instances in the life of a diplomat.