She could easily have worn red, the color of the Republican Party.

Sarah Huckabee Sanders, the governor of Arkansas, did, for her speech. So did Marjorie Taylor Greene, the congresswoman from Georgia, when she took the stage the day before. So did Katie Britt, the senator from Alabama. Their red dresses matched the glowing red ties of Marco Rubio, the senator from Florida, and Ben Carson, the former secretary of housing and urban development — worn, presumably, in honor of Donald J. Trump’s signature accessory and his tendency to dress in red, white and blue, like the flag.

But when Lara Trump, the co-chair of the Republican National Committee, delivered her keynote speech to close the second day of the Republican convention, she did so, she said, not as a party leader but as Mr. Trump’s daughter-in-law and the mother of some of his grandchildren.

And she wore black.

Specifically, she wore a sleeveless black top with a bit of satin detailing at the throat to suggest a tuxedo — as if to suggest to the watching world exactly how much could have been lost last weekend with the assassination attempt on Mr. Trump.

After all, black is an easy color to read. It means mourning. It means serious. It set a tone at a time when the question of “tone” is under close scrutiny.

Black wasn’t the predictable choice. It wasn’t what Ms. Trump had worn for her speech at the last Republican National Convention back in 2020. (Then she had chosen a particularly bright shade of carnelian.) But it was the telling choice.