Between the Chinese New Year and Valentine’s Day, red is something of the color of the moment. A week ago, however, it began to take on an entirely different meaning, as a growing number of Russians began to post pictures of themselves on social media wearing clothes in all shades of crimson in support of Yulia Navalnaya, the wife of imprisoned Russian opposition leader Aleksei A. Navalny. Red is known to be Ms. Navalnaya’s favorite color, and she wore a bright red top to her husband’s trial on Feb. 2.

As of Monday, there were 13,300 posts on Instagram of women (and a smattering of men) in red dresses, parkas, turtlenecks — pretty much any garment that could be enlisted for the cause — along with the hashtag #негрустивсебудетхорошо or “don’t be sad, everything will be OK,” which is what Mr. Navalny is reported to have said to his wife after being sentenced to more than two years in prison for a parole violation.

In the wake of the yellow vests in France, the wall of moms in yellow at the social justice marches last summer, the pro-democracy activists in black in Hong Kong, the congresswomen in white at President Trump’s 2019 State of the Union, and the women’s rights marchers in their pink pussy hats, this is yet another example of the way visual statements have become a powerful — and growing — tool of protest in the age of social media.

The literal picture of an enormous united front is among the fastest, most efficient ways to demonstrate solidarity with a cause during a time when photographs have increasingly become the currency of global communication. And nothing conveys the idea of a united front more than a mosaic of individuals in one single, bright, impossible-to-miss color.

The pro-Navalny red movement was started by Katya Fedorova, a 38-year-old fashion journalist who has worked for Russian Vogue, Interview and other outlets, and who also has a popular channel called “Good Morning, Karl!” on the Telegram messaging app.

“We’ve all been watching what was going on since Aleksei came back on the 19th,” Ms. Fedorova said by phone from Moscow. She was referring to Mr. Navalny’s return to the country after medical treatment in Germany for a poisoning that he and Western officials have described as a state assassination attempt. (It was Mr. Navalny’s time in hospital in Berlin that prompted the state’s claim of a parole violation.)

“We all knew what was going to happen, but in our hearts we still hoped,” Ms. Fedorova said. “So when it happened, I went from despair to anger, and the next morning I woke up and knew I had to do something.”

Though she had considered joining a protest, she was afraid, she said, “of being beat up or put in jail, but I have been watching what had been going on in America.” She had been particularly struck, she said, by the pictures of all the congresswomen in white, standing together against President Trump. So, Ms. Fedorova said, “even though it felt sort of stupid to use fashion, and I thought people might dismiss me, I thought an image could matter.”

Though red has certain complicated connotations in Russian history, specifically to the Communist regime, Ms. Fedorova said that to her, its significance went much further back, to the creation of Red Square and a heritage of passion and beauty.

In addition, Ms. Fedorova said she had been struck by Ms. Navalnaya’s appearance during her husband’s trial and how strong she had looked. So, after taking her daughter to school, she went home, she fished an old ribbed red sweater out of her closet and put a post up in solidarity.

“I didn’t expect more than 50 people to join me,” Ms. Fedorova said. Instead, she got thousands. Many of the posters note that while they have never wanted to be involved in politics or to speak out before, this gave them an opportunity to stand up for what they believe.

One woman, who posted a snap of herself in a burgundy polka-dot shirt, wrote: “I suggest curious people take a walk through this hashtag. It’s not about politics, it’s about solidarity and indifference.”

Another, who took a selfie in a red hoodie, wrote: “I’m a coward, I was too scared to go out into the Moscow streets on the day of the protests. And I’m not ashamed to admit it. I would be ashamed if I was indifferent about what is happening. I’m not indifferent. This post is in support of everyone whose spirit is brave and strong.”

A third, wearing a red plaid jacket, wrote: “Yes, my blog is about fashion, but I cannot stay on the side and stay silent. I don’t not care. I have been watching what is happening with horror. This photo in red is in support of @yulia_navalnaya, as well as for all who are detained and convicted, because they weren’t afraid to go out and state what they believe.”

The Navalnys’ daughter, Daria, who uses the nickname Dasha online, also posted a photograph of the whole family with herself in a red dress, in acknowledgment.

Though there has been some backlash to the red movement online, and Ms. Fedorova said she had started walking her daughter to school with her boyfriend in case something were to happen to her, she also said she was heartened by the reaction.

“To see all of us together in the same way, to open my phone and see everything red, is such a feeling,” she said. “It gives me some hope again.”