HALF A dozen teen-agers are climbing all over a scaffolding with paint cans in their hands. They are painting a wall. It will be a wall you won't forget.

"These are Hispanic and black kids from the neighborhood," says Olivia Cadaval, director of Centro de Arte, which is sponsoring the mural project along with the Latin-American Youth Center. "They're all artists."

In the middle is a huge, brilliant Aztec dragon, its paint still gleaming. Faces emerge from patterns of bright colors, and at the top someone is filling in two interlocking Olmec heads. From the dragon's mouth drip large red numbers: 2 + 2 = 4.

The numbers were put in to keep the people at the adjacent Marie Reed Learning Center happy, explains Lori Kaplan, assistant director of the Latin-American center. The wall is on a PEPCO substation at the stub end of Champlain Street. Several artists had approached the Reed school about it, but the decision lay in the hands of PEPCO, which won an award for the structure when it was built in 1926.

And PEPCO rose to the occasion. It donated not only the wall, which it had specially prepared for the work, but $530 for paint and the Aug. 5 street party that is planned when the mural is finished.

"I don't know, we might move those numbers again," mutters Big Al Carter, one of two professional artists overseeing the project. "We're talking about having a basket of fruit with numbers on the fruit. This thing keeps changing all the time."

He and Ligia Becker, the Colombian artist who started it all, keep up a running dialogue with the seven full-time and four part-time painters, ages 14 to 21. Someone didn't think the interlocking heads were multi-racial enough, but another kid said, with all those colors how could it be anything else but multi-racial? The black chain that dangles from the top looked like a slave symbol to some, but the young artist who put it there said he was thinking of the chest pendants he and his friends wear.

"We didn't want pictures of King and Malcolm X," adds Carter. "We treasure those people greatly, but the idea here is to extend our cultural horizons, do something new. This is sort of Mexican with a little bit of Matisse thrown in."

He has made other murals--as has Becker, whose most recent is at the Centro, 1470 Irving St.--because he "just likes happy things," but he also wants to do a serious mural memorial of last winter's 14th Street bridge plane crash. Right now he is simplifying elements of the colored sketch on the sidewalk while Becker works three stories up, on the rented scaffolding.

"I'm not going up there, not me," he laughs.

This is only one of several workshops held by the Centro as part of the D.C. summer youth employment program. Others are in theater, graphics, guitar, oral history and catering, for example. Funding comes from the D.C. Commission on the Arts and the private Arts D.C. The young painters get $3.35 an hour for the seven-week job.

"We started this program last summer," notes Kaplan. "We have traditionally worked with community artists."

Someone wants to know what the mural is called. "Unity," says Carter, after checking with the artists. "Except Bernard, over there. He wanted to call it 'Bernard.' "