A splash of cosmic color has come to downtown Petaluma.

With bold waves of blue, purple, red, orange, green and more, local artist Ricky Watts is transforming the southern wall of the Phoenix Theater into a huge outdoor art exhibit.

Watts describes the mural's abstract style as "space rainbows," with fluid shapes and colors that intertwine.

Work — all done with spray paint — is almost complete. Watts hopes to finish by the middle of next week.

The massive painting grew from the planning of the Cosmic Terrain art exhibition, which opens April 13 at the Petaluma Arts Center. Curator Scott Hess gathered Bay Area surreal landscape artists Mario Mars-1, Oliver Vernon, Damon Soule and Watts for the show.

"It quickly became apparent that the other three artists' works were going to take up the entire arts center," Watts said this week while on a painting break. "Rather than me showing canvas paintings, (Hess) said let's try to find a wall around town to showcase your work."

The Phoenix Theater wall was a perfect fit.

Watts, 32, grew up in Petaluma and often hung out at the theater, a hotspot for youths to gather, socialize and listen to live music.

The mural is a way to give back to the theater, which Watts said was there for him when he was "a wide-eyed mischievous teenager" spray painting graffiti. He later went to the Art Institute of California in San Diego and earned a degree in graphic design.

"The mural is something I've been dreaming of since I was a teenager hanging out at the Phoenix," he said. "It's been on my mind for probably 20 years."

Making it a reality invited several challenges. The Phoenix board of directors approved it and the city had no objection. But then, there was funding it.

Watts used Kickstarter, an online donation platform, to raise most of the project's $5,500 budget. Half of that went toward paint – about 600 spray cans of it.

Another hurdle was the sheer size of the wall. Watts had painted a 16-foot tall, 30-foot wide wall in Berkeley last year. But the Phoenix's wall is more than double that, at 40-by-60.

Watts and his assistant Warren Titus III are using a "window washing rig" lift system to access their vast canvas, which towers above an office building connected to it.

The painting itself is an adaptation of a piece called "Down the Rabbit Hole" Watts did in 2011.

"It's one of my favorite pieces that I've created in that style," he said. "That day, all the right elements were clicking and it all came together in the way I saw it happening. The art gods were looking down on me that day."

The first day of painting last week, he created a grid on the wall to plan the proportions. But the rest of the painting is done freehand.

Hess, a well-known Sonoma County photographer, described the work as "interlocking waves."

"It's a joyous uprising of color and uprising," he said, perfect for a teen center. "For a youth theater where they're full of energy and life and color, it's beautiful."

An opening reception for the Cosmic Terrain exhibit is April 13 from 4-8 p.m. at the arts center, 230 Lakeville St. in the historic railroad station.