IT'S unlikely that any of the audience members milling around the Broadway Theater after a recent preview of "The Color Purple" would have picked Allee Willis out as one of the show's three composers. Ms. Willis still wears her hair in a version of the asymmetrical style she adopted in 1981, with one side long and shaggy and the other lagging defiantly behind, ending just shy of nose-level. A pair of pink reading glasses dangled around her neck, matching her pink Keds and the pattern of pink bubbles that floated all over her loose shirt.

With her cheerful, kooky aesthetic, Ms. Willis looked like the vision of an artist that a children's book illustrator might put on paper; she didn't, however, look like the kind of slick professional one imagines a producer would seek out for a $10 million Broadway-bound production like "The Color Purple," an adaptation of the Alice Walker novel with the high-power blessing of Oprah Winfrey. (The show opens on Thursday.)

Nor do Ms. Willis's credentials make her an obvious choice. "I have zero musical training," she confessed while seated in the theater's downstairs lobby. "Zero."

That lack of formal education didn't stop her from becoming a success in the pop music business, the songwriter behind hits like the Pointer Sisters' "Neutron Dance," Earth Wind & Fire's "Boogie Wonderland" and the lyrics to the theme song from the television show "Friends." Even her old friend Scott Sanders, the lead producer of "The Color Purple," didn't think of her as a possibility when he started scouting for songwriters for the show, about an abused African-American girl who struggles to find independence.

In recent years Ms. Willis, tired of the grind of the music industry, had been finding creative outlets elsewhere: creating an alternative cyberspace for Intel; painting kitsch-inspired art (which turned out to be a big business); building motorized sculptures, to considerable acclaim; and art directing the occasional music video. As a result, when Mr. Sanders called her in 2001, he was seeking advice rather than offering a job: unsatisfied with a slew of demos he had heard from other musicians, Mr. Sanders wanted to know her opinion of Brenda Russell, an R&B artist he thought he might approach.

Ms. Russell, like Ms. Willis, was a successful 80's songwriter, though her music fell on the jazzier, more soulful side of the spectrum. (She was nominated for a Grammy in 1988 for the single "Piano in the Dark.")

By sheer coincidence, at that very moment, Ms. Russell was driving over to Ms. Willis's house in Los Angeles. She, Ms. Willis and another pop musician, Stephen Bray, a former boyfriend of Madonna's who co-wrote and co-produced hits like "True Blue" and "Express Yourself," had been collaborating for months on some relatively obscure projects that were, as Mr. Bray put it, "more a social thing than work." They were writing the music to accompany two animated series, "Driving While Black," which appeared on urbanentertainment.com, and "Fat Girl," which ran on Oxygen, both collaborations with Prudence Fenton, the animator who had long worked on "Pee-wee's Playhouse." (Ms. Fenton and Paul Reubens, a k a Pee-wee Herman, are close friends of Ms. Willis's.)

Ms. Willis, after the way the three disparate musicians collaborated on those smaller projects, proposed to Mr. Sanders that the threesome write a few songs on spec, an unusual arrangement for songwriters at their professional level. A few months later, they landed the job.

For Mr. Sanders, it was a risky choice: not one of the three 80's-era pop songwriters had ever written for musical theater before. It is not unheard of for pop musicians to cross over to musical theater - Elton John's work on "The Lion King" and "Aida" are probably the best-known examples. But until starting to write for "The Color Purple," Ms. Willis's familiarity with that genre extended no further than what she had picked up in the late 70's as a hatcheck girl at the New York cabaret club Reno Sweeney, where Bette Midler performed. Mr. Bray and Ms. Russell, at least, grew up hearing show tunes.

"Instead of listening to middle-class black-people music, my parents would literally stand around the piano singing, 'When the children are asleep...,' " crooned Mr. Bray, having joined Ms. Willis, along with Ms. Russell, at the Broadway Theater that afternoon. "It was totally 'Ozzie and Harriet.' "

Ms. Willis, the team's one white songwriter, laughed. "And all the black music was happening at my house," she said. Ms. Willis, like Mr. Bray, grew up in Detroit, a few miles from the two houses that then constituted Motown. While Mr. Bray was singing Rodgers and Hammerstein with his parents, Ms. Willis was listening intently to the background vocals and bass lines she could hear emanating out of the Motown studios she frequently visited.

Much of the process of writing for theater was new and intriguing to the trio. "I'm not used to people telling me how to rewrite a song," said Ms. Russell, explaining that in pop, a song is either recorded as is or rejected. "This opens this other door of changing and reworking and collaborating."

They also had to adjust to the plot-driven and visual demands of writing songs for theater. "Pop songs make a point, and then they keep finding different ways to reiterate the same point," Ms. Willis said. (To wit: "I'll Be There for You," the "Friends" theme song.)

Mr. Bray picked up her thought and finished it: "In theater, when you hear repetition, it's like, 'You said that a minute ago, why are you saying it again?' It's really not O.K. The lyrics need to be action driven, involve some behavior - it can't all be interior." All in all, the team wrote close to 20 songs that never made it into the play for one reason or another.

The group initially worked at Ms. Willis's home, where Ms. Willis, who owns one of the largest kitsch collections in the world, redecorated her home to show off her African-American treasures, trying to set the right creative environment for the show, which takes place from 1909 to 1949. Among the objects on display: a Louis Armstrong statue, percussion instruments from the Cotton Club, matchbooks with Zora Neale Hurston's picture and hand-held church fans.

The three worked in their idiosyncratic style, mixing high-tech tools - Ms. Willis's 17 networked Macs, which they used for research, and programs that allowed them to digitally record complete orchestrations - and very low-tech instruments like an old manual eggbeater or sandpaper - "anything that might inspire an idea for a rhythm," Ms. Willis said.

They resisted picking any one musical genre to define the show. "Songs that started out as gospel and ended up as gospel seemed flat to me," Ms. Willis said. "We didn't want it to sound like a revue. Using the different characters, we wanted to combine different styles of the time. The challenge was how to put our stamp on this without being blasphemous to period music." Blues, modern pop, funk and gospel sounds are mixed throughout the score, sometimes within the same song.

Over four-and-a-half years their method of collaboration developed into a process of unpredictable juxtapositions. Sometimes they used some chords that Ms. Russell played on the piano as a starting point for a mood; other times Ms. Willis might pick up a copy of the novel and start putting melody to actual words on the page. All three wrote lyrics and melody, sometimes simultaneously, improvising along with a riff one had started. Ms. Willis recorded everything, wanting to be able to return to fresh moments of creativity should something become lost in the process.

"I'm very much a fan of spontaneity," Ms. Willis said. "I'm also an archivist. I'm more interested in recording how someone goes through the day than the day itself." As a result, they have on digital audio the moment when Ms. Willis unexpectedly found a way to work the name of the play into an anthem for the finale, a title they had not planned to insert.

On that day, Ms. Willis recalled, she felt the lyrics coming on, and announced, "O.K., guys, get ready, or take a Valium."

Holding the novel in one hand, she started to sing, "Like a blade of corn, all a part of me/ Like the color purple, where do it come from?"

The next sound on the digital audio recording is the noise of all three of them screaming, knowing they had hit on what they needed to close the show.

"This was not the kind of songwriting I'd ever done," Ms. Willis said, "but it feels so natural. You couldn't pay me to go back to the other way."