MODELING ON THE PAST

They laughed at ''government green.'' In the design schools, they taught the morality of primary colors - red, yellow, blue as flags of aesthetic honor - and history teachers chided the gullible in government who had slapped an institutional green on every surface in the mistaken notion that the shade offered ''eye ease.''

Pilasters stuck to interior walls evoked equal mirth, if not outrage. Columns in the orders memorized by school children (Ionic, Doric, Corinthian) struck them as fossilized. For wasn't history, as H.L. Mencken had railed, bunk to the architectural iconoclasts of the modern movement?

No surprise, then, that when a classical concoction hit the landscape it seemed simply silly. J. Paul Getty's $17 million museum modeled on a Herculaneum villa in Malibu, Calif., ''bemused'' a Los Angeles critic and his peers. It would put future archaeologists in a fog, he wrote:

''From the fluted columns, the Doric and Corinthian capitals, the tiled mosaic floors, the bronze and marble sculptures, the layout of peristyles, loggias, temples, and cubicles, they will conclude that the villa was built in the first century before Christ, perhaps by Julius Caesar himself, who could easily have subdued the local savages.''

It is less than a decade later now; in a period dubbed post-modern, the popular museum has, in fact, begun to select three sites to expand. And, in our outrageous, eclectic era, tints of pink and ''government green'' adorn trendy offices; high-style chambers display pilasters and columns; and Beaux Arts-educated designers, once relegated to the attic for the sin of fogyism, are set to work at '30s-style renderings.

Old arguments that called classical architecture an applique - below a paste-on pediment, above a turret - are still now.

Wherein was it written, architects now ask, that greater virtue lies in the crisp cube than in the sinuous acanthus leaf of the ancients climbing up a capital?

''How would a Roman go about adding space?'' was the question architect Hugh Hardy of Hardy Holzman Pfeiffer Associates asked director Robert C. Doty at the Currier Gallery in Manchester, N. H.

With two wings, one to either side, in an unabashedly neoclassical addition, was Hardy's answer.

And what would a Roman call these wings, flanking Edward Tilton's 1929 original? ''Pavilions,'' naturally, says Doty. ''A true Palladian concept.''

How do you obey Roman design formulas? the architect went on. With a formal symmetrical entrance, columned windows, an arched entry, and pale-beige brick that matches the original central limestone structure. . . .

If New York architects don't wear togas, and their clients can't afford the craftsmanly trim of architects copying classical lines of decades past, the Currier Gallery has used such responses to secure a serviceable Beaux Arts-plan building within and a good neighborhood match without.

The two wings accommodate a 10-foot slope in the site. The natural light enters through windows and skylights that parallel or fit into the old structure and the carved facade of that structure is used to make a handsome wall in the new space.

But in architecture, as in life and literature, you cannot always go home so easily.

Take the rules for the handicapped. To secure federal funds, any new building requires wheelchair access. In some buildings, this has demanded costly excavation; in others, tortuous ramps. Here it required an heroic labor to integrate five stepped levels in the old museum.

This meant, for instance, that the front courtyard, for all its opening embrace of space, rolls down driveway-style into a turnabout that looks rather amiss, especially with the contrast of its craggy paving blocks and bollards against the slicker surfaces of the addition.

Somehow, then, for all the touches of reverence for the past - the attention to detail in the brick coursing across the facade; the rounded, ridged columns; the contoured ceiling of the gallery space - the new building doesn't quite measure up.

Much as a de Chirico or Magritte painting of a building deliberately eschews the ornament that breaks down scale in order to heighten the monstrousness of an imperial architecture, so today's lack of detail gives this neoclassical conception a surreal air.

Of course it is not just in Manchester that something is askew in this replay of classical design. Sometimes, it is through indifference. (Certainly Philip Johnson did not fret that his AT&T pediment would be invisible from Manhattan streets.) Sometimes it is deliberately off (the delightful play with window scale at the Nantucket and Block Island homes by the firm of Venturi, Rauch & Scott Brown). More and more, however, the banal re-rendering of the past is dreary.

''Calculated naivete and sometimes even a knowing ugliness,'' Christopher Jencks puts it in his compendium of ''Post-Modern Classicism.'' ''We may, at best, be on the verge of a mini-Renaissance.'' At their ''best,'' fan-light windows and such can be fresh and fun.''Private Jokes in Public Places,'' Moshe Safdie's recent book countered. Walk through an architecture design studio, he suggested. A student ''project might be a little museum in Cambridge whose ground floor would be built with masonry, as if it had existed for some time, and whose upper floor would be built with sheet metal, suggesting that it was a later, temporary addition.''

Ironically, the Currier Gallery addition does suggest just that. The aluminum roof atop the solid pavilions outside and the trussed ceiling inside mix high tech and not-so-high historicizing with less than striking success.

For all such awkwardness, it is not that the Hardy addition is less than competent, or that the Beaux Arts plan won't work. Quite the contrary.

Perhaps we simply lack the patience to wait for such amenities as boxwood to age in symmetrical splendor or the funds to plant gardens as the Getty has. Hence, we must suffer the graceless boulders and yews of an abysmal landscaping that emphasizes the contrast with the past.Perhaps, too, we no longer have the resources to create tile roofs and nicely limned limestone facades. In that case , though, we need to rethink such modeling on the past before we turn its architectural evidences into trade-magazine covers or magnets for the busload of Manhattanites who came here to visit last month.

''Cleanse out the old leaven that you may be fresh dough,'' declares the sign before a church a few blocks away from the Currier. While one welcomes the new classicists' remaking of old ''dough,'' one still waits and wonders whether the mold is really viable today.