A subdued J. Marshall Coleman, his political dreams shattered but his wry sense of humor intact, emerged from seclusion today to discuss his crushing loss to Charles S. Robb in Tuesday's election for governor of Virginia.

With his wife Niki at his side, Coleman told a news conference he took full responsibility for his defeat, the first time in 16 years that a Republican has lost the governor's race. "I was the head of the ticket," he said. "It was my responsibility to win the election and I didn't do it."

Until his defeat, Coleman was considered one of the GOP's brightest young stars. Now the 39-year-old lawyer's political career appears all but over.

Coleman, who had never lost an election before in 10 years of public life, said today he had no idea what he would do next although he ruled out running for the U.S. Senate next year. "I didn't expect to be having to make plans this way and I made none," he said.

There was little or no outward bitterness in Coleman's analysis of his defeat and much of the wit that he sometimes had suppressed on the campaign trail. He mocked his own polls, which had showed him overcoming Robb last weekend, telling reporters "I know you think we're still trailing but we've got this overnight tracking sample that shows us ahead."

Later he quoted Winston Churchill, who on the night of an election loss told his wife that if his defeat were a blessing, "it's most effectively disguised."

Gone was the conservative rep tie and white shirt that Coleman had sported throughout the campaign, and that had contributed to the notion that he and the identically dressed Robb were little more than "Tweedledum and Tweedledee." Today Coleman wore a dark green tie with no stripes and a blue-striped shirt.

Coleman refused to lay any blame on his running mate for lieutenant governor, state Sen. Nathan H. Miller, the subject of conflict-of-interest charges that devastated Miller's campaign and put Coleman's on the defensive for weeks. Nor did he criticize former Gov. Mills E. Godwin, whose lukewarm support cost him the votes of many old-line conservatives and whose last-minute attack on Robb on a series of racially tinged issues may have contributed to a massive turnout by blacks who voted overwhelmingly Democratic.

Coleman said he was "astonished" by Robb's charge that the GOP was making a racist appeal in attacking the Democrat's support of the Voting Rights Act, postcard registration of voters and congressional representation for the predominantly black District of Columbia. "I didn't think it was a fair shot," Coleman said.

Otherwise, Coleman had no bones to pick with Robb, whom he had vigorously attacked throughout the race as a big-spending Great Society liberal. "He was a very attractive candidate, exceedingly well-known," said Coleman. "We started out against formidable odds," he added, referring to Robb's early lead in the polls.

Coleman said he was disappointed that blacks, who had given him nearly one-third of their votes in his successful race for state attorney general in 1977, had turned out in record numbers for Robb this time. He asserted that his constant emphasis of his support for President Reagan was a plus for his campaign, but conceded that it helped trigger the black reaction. "There is some open hostility in the black community for the Reagan program," he said.

He denied he had retooled his political beliefs for this year's race, moving from progressive Republicanism to the Reagan-style conservatism. But, he conceded, "clearly the perception of me was a different one in 1977 than in 1981."

Coleman said he had received phone calls from the president, Vice President Bush and Sen. Harry F. Byrd Jr. commiserating over his loss, but no call from Godwin.

And with that, the candidate returned to his campaign headquarters, where workers have begun removing the furniture and staff members were searching newspaper want ads for new jobs.