WASHINGTON doffed all those tired winter greys and burst into dazzling technicolor right on cue yesterday for its 180th birthday celebration at city hall. But the celebrating isn't over yet. Until the end of September, an official committee of dignitaries and people that mayors always name to these groups in an election year will be thumping the tubs for the tourist trade.

City fathers over the four score and 100 years of Washington's life have formed such committees. A year ago, it was Mayor Barry's turn, which prompted some speculation in this space that the committee might recommend "a fundamental approach to improved tourism in the nation's capital... linked to the maximum extent possible with attractions that lure people into the central city and its surrounding metropolitan areas."

But there's nothing wrong with a promotional campaign. Nor does it hurt to spark civic pride around town, which is what this latest committee intends to do, so long as it doesn't bog down in draft reports, reviews, background papers and other boring bits of boosterism.

After a sharp drop in business over the winter, hotel operators are reporting a vigorous upturn. For that drop, according to Carole Shifrin in the Washington Business section, local officials blame terrible weather around the country, pressures of the economy and federal budget cuts. In January 1981, the occupancy rate was helped by an inaugural, the return of Congress nearly three weeks earlier than this year and a smaller supply of hotel rooms. The Jan. 13 Air Florida crash certainly didn't help this year.

In addition to promotions, Washington also needs an all-out improvement of the information it offers visitors:

* Metro must keep simplifying its bus maps, post more information at stops, reprimand bus drivers who glower and mumble, and stock every hotel with the most complete guide to public transit.

* The city should distribute more advice and warnings on shoddy cab service. In New York, cards at the airports describe in five languages the fares, rules and ways to report complaints.

* Street signs are still terrible. The human eye cannot simultaneously watch the road around an unfamiliar circle and read three different narratives per light about right-turn possibilities, lanes usable at a given hour and one-way changes.