That was the story on the South Lawn of the White House yesterday as dusk descended and the whir of a generator energized dozens of spotlights bouncing colors off the traditionally staid white walls of the traditionally staid old White House. Tourists and hometowners driving by probably did double takes as they glanced casually at The Green House . . . wait a minute, that's The Blue House . . . Big Pink . . . Little Red . . . Las Vegas on the Potomac. Last night, with the Reagans wisely out of town, 1600 Pennsylvania Avenue was The Light House.

Nancy Reagan's decorating revenge? Merry Pranksters infiltrating the Secret Service? No, the White House was posing for a picture, a commercial picture at that. The White House, notoriously camera-shy, allowed itself to be talked into a session with New York photographer Dudley Gray.

Now, Gray is not your average photographer; he's an illuminator, a portrait artist who builds temporary light rigs around the huge structures he so passionately wants to bend to his vision. Gray has already illuminated the top of the Empire State Building, the Brooklyn Bridge and City Hall in New York. Yesterday the commercial artist, working on New York time, met the bureaucracy, working on Washington time. Guess who won.

"Are you shooting, Dudley?"

"A green, a green, a green, Clyde!"

"I'm ready now, please, hold it, please!"

Time started running out on Gray and his crew of 14 before they ever got off a shot. With lighting partner Joe Strand and a hastily assembled light system, Gray started setting up on the South Lawn late yesterday afternoon. The spots were spread wide across the lawn, as if the still-White House would hold its own press conference. The walls may once have had ears, but the only mouth working was Gray's.

"We're lighting the trees, the branches; soon the whole thing will be burning, the bushes will be burning. We'll bring the whole thing up for a climax." If he weren't such a good photographer, Gray would make a good dilettante. He talked about needing a tape of singing whales for inspiration; about the fact that he'd had 12 hours to light the Brooklyn Bridge; about the intransigence of White House press aide Sue Mathis, who obviously regretted the entire affair.

Meanwhile, the White House stood in mute eloquence as different gels (the filters over the lights) created layers of colors and crossing shadows, accompanied by half-muttered curses and the sound of glass filters breaking on the lawn; the filters faltered under the intense heat of the lights. It's a good thing no one's going to be running around barefoot on the South Lawn any time soon.

Gray drifted around, shooting from his Nikon F-2, while Strand shot from a Polaroid and someone else shot from a 4-x-5 camera. They expected to make between 2,000 and 5,000 shots, of which one or possibly two will be selected for a monstrous (and probably expensive) Washington, D.C. book, due to be published by Harry N. Abrams in fall of 1982. The book will feature more than 200 photographs from the likes of Ernst Haas, Jay Maisel, Fred Maroon, Fred Ward and National Geographic's Bill Weems and Sosee Brimberg. Gray's two-hour shoot (he had to finish by 9 p.m.) is expected to cost $6,000, with donated products and services from the private sector helping to defray expenses. The coffee-table book will be designed by J.C. Suares, the former art director of New York magazine, who recently put together Abrams' $45 opus, "Manhattan."

Several guards at the South Portico entrance had to move away from their post, being literally blinded by the lights. They seemed to be the only ones half impressed by the apparently scattered genius at work there in the night; in truth, Gray seemed to be making it all up as he went along. The colors were pretty but ultimately boring, as if the troupe were lighting up the set for a remake of "Mr. Smith Goes to Washington" . . . only the actor was out of town.

"Wait, wait, two more gels . . . we're going to have one big hit! There's just not enough time in the day . . . they don't understand." Gray moaned and shot, moaned and shot, moving around to different vantage points, seemingly pleased with what he saw in his mind's and camera's eye. For her part, Mathis didn't seem to think the project was such a bright idea, despite the 50,000 watts of power needed to illuminate Gray's vision. Only time will tell, though only Newsweek sent a photographer to shoot the shooting.