LAST JULY TWO weeks before Jummy Carter chose Walter Mondale as his running-mate, the Minnesotan rhapsodized in an interview about some of the joys that would be his if Mr. Carter chose him - and the public, in turn, chose them. Among those joys he listed the prospect of moving into the Naval Observatory midway up the hill on Massachuseetts Avenue NW. And he went so far as to promise - without reservation or qualification - that if elected he would make it his business early in his administration to make a coating of white paint to the two heavy anchors decorating the obsevatory's entrance. For reasons unknown, during the Rockfeller residency they had been apinted a Stygian black. Although we haven't gone through wvery syllable of the Mondale record since that day, we beleive that this was his sole campaign pledge made independently of Mr. Carter.

So we are pleased to report that this pledge has been kept - comfortably within the first 100 days. True, the Vice President did seem for a time to be waffling, in a way that gave a lot of us some nervous moments. Two weeks ago, commuters were startled to find the achors a seasick green. Days later, they were school-bus yellaow. But now, wer are pleased to report, they are white - a sparkling white. And that, we have since been assured, was the intention all along. The green and yellow periods were never anything more than undercoatings, necessary because the achors had been corroded over the years.

The newly whitened anchors are both appropriate and functional. They are, you could say, Navy white, which ought to please the former-midshipman-become-President. And because they gleam smartly, they serve as useful landmarks when locals give out directions - as in, veer left 20 yards past the white anchorss to get to lower Wisconsin Avenue, or turn hard right at the anchors for Reno Road. They have, moreover, a deeper significance: By making good on his bold pledge, Mr. Mondale may have become the first major candidate ih history to meet his obligation to the public by applying a quick and complete whitewash.