Actor and salad-dressing maker Paul Newman plans to build a $6 million outdoor camp for children with life-threatening diseases on a 400-acre YMCA site in Connecticut, Ann Reznikoff, a spokeswoman for Newman, said yesterday. She said that plans for the camp are still "embryonic" and many details still unclear.

Ursala Hotchner, an executive with Newman's Own Salad King food company in Westport, Conn., said the actor was approached recently by the Torrington-area YMCA for help in refurbishing an old 400-acre camp near an existing YMCA day camp. Construction is set to begin in the spring of 1986 and the first campers are scheduled to attend by the summer of 1987. The camp will be known as the Newman's Own Hole in the Wall Camp, and children attending the YMCA day camp may share some facilities with the Newman camp.

YMCA Executive Director Tom Colligan said the camp will serve ambulatory children with ailments such as congenital heart disease, hemophilia and cancer. Rosalynn Carter's Forum

Former first lady Rosalynn Carter kicked off her first symposium on mental health policy yesterday in Atlanta, listening as George Gerbner, dean of the communications school at the University of Pennsylvania, blasted the media's portrayal of the mentally ill.

"Stigmatization used to be done by ancient and medieval religion," said Gerbner. "Today it's done by the mass media." He accused the media of often projecting an inaccurate image of the mentally ill.

The symposium, the first of four annual meetings to be chaired by Carter, is sponsored by Emory University's psychiatry department with a $100,000 grant from the Gannett Foundation. "I've been looking forward to today and becoming more active in the cause of good mental health," said Carter, who chaired the President's Commission on Mental Health during the Carter administration. During the discussions, she sat in the front row beside her husband, Jimmy Carter, who said he had "come here as an interested spouse." End Notes

At a meeting Thursday, the 100 members of the Collectors Committee of the National Gallery announced their purchase of an abstract painting by California artist Sam Francis. The work, titled "White Line," was purchased for slightly less than $700,000 and is on display in the East Building of the gallery . . .

Helen Thomas, White House bureau chief for United Press International, A.M. Rosenthal, executive editor of The New York Times, and Keith Fuller, former president and general manager of the Associated Press, yesterday were named Fellows of the Society, the highest honor given by the Society of Professional Journalists, Sigma Delta Chi. New society officers are Robert Lewis, Washington reporter for Newhouse News Service, president; Robert Wills, editor of The Milwaukee Sentinel, president-elect; James Plante, managing director of news support services for NBC News in New York, secretary; and Paul Davis, news director of WGN-TV in Chicago, treasurer . . .

Composer Aaron Copland celebrated his 85th birthday Thursday night at Lincoln Center in New York City listening to his own music. In a speech during intermission, conductor Leonard Bernstein recalled his first meeting with Copland on the composer's 37th birthday, when Copland sat next to him at a dance premiere. "Aaron Copland makes Americans feel American, whoever we are, whatever our roots are," Bernstein said . . .

Salvador Dali, 81 and ailing, has agreed to design a 9,200-square-yard plaza for Spain's capital city. The centerpiece of the Madrid plaza is to be a 230-ton granite monument to Isaac Newton, a spokesman for the painter said. Dali has not appeared in public since he left a Barcelona hospital in October 1984 following hospitalization for burns he suffered in a fire . . .

The possible rematch between newly crowned world chess champion Gary Kasparov and former titleholder Anatoly Karpov would be in February, World Chess Federation President Florencio Campomanes said yesterday. Karpov, in an interview shortly after he lost the title a week ago, said he hasn't decided if he'll accept the rematch that Kasparov must offer. If he does, the match again will be limited to 24 games, Campomanes said.