Kerry Washington’s true colors came out over a pink cardboard box of croissants.

“You have to take them back to your office,” said Washington, 46, as she sat, cross-legged, on a wingback chair at the Spence School on Manhattan’s Upper East Side. “No, really. You must.”

We’d already located her graduation picture on the wall (Class of 1994), marveled at the weather (blistering) and admired the drawing room’s décor (Oval Office meets “Masterpiece Theater”). We laughed when the person who delivered the pastries referred to them as “performative croissants.” Nobody eats during an interview; refreshments are part of the set.

The point is, Washington sealed the fate of those baked goods with a conviction and calm that set the tone for the next 90 minutes, even when the conversation turned to heavier topics. (Spoiler: The croissants did not go to waste. They were on a downtown No. 6 train by noon.)

For years, Washington has guarded her personal life with the same tenacity she’s breathed into characters like Olivia Pope on “Scandal” and Mia Warren in “Little Fires Everywhere.” Her June 2013 wedding to Nnamdi Asomugha was a secret. She never posts pictures of their three children. She was the one who chose her alma mater as a venue; she wasn’t going to invite an inquisitive stranger to her home in Los Angeles.

Now, with a memoir coming out on Sept. 26, the Emmy-winning actor is opening the door to her inner sanctum. “Thicker Than Water” tells the story of a Black girl from the Bronx making her way in white Hollywood while feeling as if she didn’t belong in her own family.

Before we delve into the reason for this disconnect, let’s give credit where credit is due: Shonda Rhimes advised her friend to write a book years ago. “You grow as a person just from the act of writing,” said the renowned producer and “Scandal” creator, whose memoir came out in 2015. “It’s a powerful method of reclaiming yourself.”

But Washington felt then that she was too young to take stock of her life. She said: “I always had this nagging sense that there was something fraudulent about it. I didn’t quite know myself well enough.”

In early 2018, after seven seasons on “Scandal,” Washington filmed the final scenes of the show. (Because of the SAG-AFTRA strike, she was careful not to say its name during our meeting.) She planned to write a book of lessons learned from Olivia Pope, the fictional Beltway fixer inspired by the real-life crisis manager Judy Smith, who made Washington the first Black woman to lead a network drama since 1974.

“There was a sense of completion,” Washington said. There was also a sense of readiness — for new projects, adventure, the unexpected.

It seemed like the perfect time to learn about her ancestry, so Washington agreed to appear on Henry Louis Gates Jr.’s PBS series “Finding Your Roots.” In order for research to start, she needed DNA samples from her parents, both in their 70s.

“When I said, ‘Spit in this tube,’ they started freaking out,” Washington said. “My mom was like, ‘I didn’t know that this was going to happen.’”

Soon after that, her father started having panic attacks. Thinking his breathing difficulties and insomnia might be related to anxiety about the bad behavior of a soon-to-be uncovered relative, Washington told Gates she had to back out of the show. “I said: ‘I just don’t think they’re going to agree to it. My dad’s really uncomfortable.’”

Gates offered to speak with her parents privately, to assuage their fears. Months later, Washington learned a bit about the conversation from Gates. “They asked Skip, trying to sound, I think, as if it was a hypothetical: ‘Let’s say there was a possibility that she wasn’t our biological child. Would that come up in the testing?’” He assured them that it would.

Without explicitly telling her parents what to do, Gates recommended divulging such information while all parties were still alive.

On April 3, 2018, Valerie and Earl Washington finally shared the secret they’d kept from their only child for more than four decades: They had used a sperm donor to conceive her.

Washington’s initial response was a combination of excitement and liberation. Now, at least, she had an explanation for her struggle with, as she put it, “not quite belonging.”

She said: “I’ve always had this weird disconnect with my dad, but I thought that was my fault. I thought I wasn’t a kind enough person. But the idea that I was not his never occurred to me. It was just, why can’t I be better to him? Why can’t we be closer? What’s wrong with me? What’s wrong with us?”

But, Washington went on, “I immediately felt guilty because I could see how much pain my parents were in, my dad in particular.”

Her empathy came with a side of resentment. “I was birthed into a lie,” she said. “I was playing a supporting character in my parents’ story.”

Washington added: “It felt like I’d been wandering through a library my whole life, looking for a specific book about myself. My mom and dad were these librarians who said, ‘There’s a room we haven’t shown you.’”

By now we’ve all come across sitcoms, movies, documentaries, podcasts and books inspired by 23andMe discoveries. Odds are, you know someone who’s made a deposit or a withdrawal from a sperm bank. Maybe you’ve helped select a donor with 20/20 vision or a Ph.D.

But the mid-1970s were a different time. As Washington pointed out, her mother’s doctor on the Upper East Side probably could have counted on one hand the number of Black women who walked into his office to discuss donor insemination. The Washingtons requested a donor who was healthy and Black. (Despite Kerry Washington’s best efforts, there remains no way to determine his identity.)

“I know that their intention was to protect me, to love me, to take care of me, to keep my world simple,” Washington said. “I get many years of not telling me, but I’ve been an adult for over two decades.”

Her mother explained that there never seemed to be a good time. Washington understood: “When I was in treatment for my eating disorder, didn’t seem like a good time to drop a bomb like that. Then I was in a tumultuous relationship, didn’t seem like a good time to drop a bomb like that. Then I was having my own kids — her intention was in the right place.”

The plan, her mother said, was to leave a note in a safe deposit box.

“I was like, ‘You’ve had cancer three times and you’re knocking on 80,’” Washington recalled telling her mother. “‘At what point were you going to write that note?’”

She added, “I will be forever grateful to Skip Gates.”

As she processed the news, Washington tried to “plow through” life, “as high-functioning people do.” She kept appointments, cared for her family and generally did what was required of her. But Washington’s autopilot had its limits. The book she’d signed up to write suddenly seemed as if it belonged in a different library.

“Any attempt I made to sit down and write about my life and not include this new information, it just felt impossible,” she said. “I tried to give the publisher their money back.”

Tracy Behar, Washington’s editor at Little, Brown Spark, encouraged her to put the idea aside and take some time to think. “Six months or maybe a year later, she came back and said, ‘I want to write an intimate family memoir,’” Behar said.

Washington started by recording memories on her phone. Then she began writing — no more than 1,500 words a day, composed while standing at an island in her closet with a view of shoes and sweatshirts. Washington did not have a co-author: “Having it sound like me felt really important.”

She wrote about her parents’ fighting, her father’s drinking and other secrets that crowded the family’s apartment on Pugsley Avenue: her father’s legal troubles, her mother’s first marriage and stillbirth, her own fear and confusion after being sexually abused by a family friend. When Washington confronted the boy, he told her she was “crazy.” Later, he stopped after she threatened to expose him.

She wrote about finding solace at a local pool — “Being in the water, moving through the water has always felt more natural to me than walking on land” — and becoming a formidable student. By the time Washington arrived at Spence, an hourlong commute and a world away from home, she was an experienced performer in every sense of the word. She suffered from insomnia, depression and an eating disorder. She never felt safe, but always soldiered on. By the time she arrived at George Washington University, she was an actor. You can find the details on IMDb. The rest — the important stuff — is in her memoir. It has Washington’s restraint while being both sincere and witty, a rare combo.

Given the origin story of “Thicker Than Water,” the actor’s own origin story is a fairly small part of the book. What’s exciting is watching Washington figure out for herself that the sperm donor doesn’t even rank in the top 10 most interesting things about her life.

“I’m still missing this piece of not knowing where half of my biology comes from,” she said. “At least I don’t have any of the wrong pieces in the puzzle anymore.”

For the cover, Washington proposed the idea of being shot underwater, and Behar thought of Reisha Perlmutter, a fine artist specializing in aquatic painting.

For her first-ever book project, Perlmutter took around 10,000 pictures of Washington in her parents’ pool and then created a portrait of her face peering at her own reflection. The effect is reminiscent of a 1980s school picture where the subject’s profile hovers in the background — only in this case, the spectral image merges with the one facing the world. The first is haunting; the second is hauntingly beautiful.

Perlmutter said the approach “worked well for Kerry’s memoir about going deeper into herself.”

Washington said her parents “aren’t doing somersaults” about the book, “but they’re supportive.” When asked if they would be willing to discuss it, she said, “I think they want to be in the wings on this. Even though it is the story of our family, they would each have written a different book.”

Valerie Washington, a retired professor, returned an early manuscript marked up with red pen. “Some of it was grammatical,” Kerry Washington said. “Some of it was little inaccuracies. It was this street, not that street; this beach, not that beach.” Her mother said she was proud.

In late July, Earl Washington was still working his way through the memoir. “It’s been hard,” Washington said. “He does a lot of processing with my husband.”

Washington recalled a conversation with her father in which she told him: “‘I’m not going anywhere, you’re not going anywhere, you’re my dad. Now when I say I love you, it’s not because of who you’re pretending to be in my life, it’s because of who you are.’”

Once her book comes out, Washington is aware that she may be inundated with stories from readers who see themselves in her, and want to talk about their families’ secrets. She appeared unfazed by the prospect. “I never want to say to people, ‘You have to tell your kid the truth,’” she said. “I do think it’s extraordinary how few rights I have as a donor kid. But that dissonance thing is something I want us to be aware of. To know that when we cause a person not to trust their instincts, we take away some major tools they have to operate in the world as confident people.”

By writing “Thicker Than Water,” Washington appears to have reclaimed those tools.

Audio produced by Adrienne Hurst.