Last night -- when she knew all of us would be looking very hard -- Barbara Bush chose to wear a sapphire blue dress by Arnold Scaasi. It had a simple, low, square neckline. Long fitted sleeves. A velvet bodice. A full satin skirt draped asymmetrically, pulled to one side with a bow. She wore diamond drop earrings and her trademark pearls, and had her hair swept back. But clothes aren't what the new first lady is all about, even if hers will someday end up in the Smithsonian. Her style is Yankee no-nonsense. She's a woman who doesn't need to wear frills or makeup to look elegant, and she knows this. "Look at me good this week," she said Thursday, "because it's the only week." Barbara Bush makes us feel almost embarrassed to be paying so much attention. Not that she looked uncomfortable in her Scaasi -- or in the simple Bill Blass teal-blue wool coat she wore to the swearing-in ceremony, or the collarless white wool dress underneath. It's just that she seems uncomfortable with the subject of fashion. "I don't think clothes will be her preoccupation," said Blass -- who was surprised and pleased yesterday morning to see her on television wearing his suit. He loved her white gloves too. "Very crisp. Very refreshing," he said. Yesterday she didn't wear the pillbox hat that was made to match her coat, maybe to downplay the outfit, maybe when she found out Marilyn Quayle was wearing one. Or maybe she knew it would blow off in the wind, as Quayle's did when the Reagans' helicopter took off. And Bush unabashedly wore the same three-strand pearl choker day and night. It's fake. "I don't think she's downplaying her looks," Scaasi said yesterday. "I think that she's very up about herself now -- and how great she looks ... She's a real lady, and I think more than anything, that comes off." This much is certain: She likes solid, bright colors (maybe a preppie holdover), and blue in particular. After eight years of Nancy Reagan's red -- which seemed so shockingly bright at first -- our eyes seem to have adjusted. These hues are almost expected now from the first lady, and from women in politics generally. They're mediagenic. They're flattering. And they show up well at a distance. And this much more is certain: She likes Scaasi. He designed all three evening gowns she wore this week. He designed the red wool suit she topped with a stars-and-stripes scarf at the Lincoln Memorial ceremony Wednesday. And he designed the blue wool suit she wore to her Kennedy Center tribute on Thursday. That was when she said to the audience, "Please notice -- hairdo, makeup, designer dress." Like his wife, President Bush seems to have made an effort to tone down the glamor, the formality. He chose to wear a dark suit with a silvery gray necktie to his swearing-in. In 1981 President Reagan created a stir by returning to a formal morning suit for his swearing-in, a tradition that Presidents Carter and Johnson had dismissed. Barbara Bush's evening dresses could not be described as low-key, really. But within the Scaasi collection she chose simply -- he is capable of much fancier, frillier designs. At the dinners Wednesday night Bush wore a gown with a full black silk satin skirt and a white bodice with diamante' swirls. It was from Scaasi's ready-to-wear collection, which retails from $1,000 to $3,000. To the gala she wore a made-to-order Scaasi (as was her inaugural ball gown) -- these are designs that retail for about double the ready-to-wear. The gala dress had a white satin skirt and a red reembroidered lace bodice applique'ed with black velvet bows. Marilyn Quayle, on the other hand, may eventually be more identified with her clothing. She's willing to stand out, which means she's probably willing to be criticized. Just as Nancy Reagan risked it when she wore her little red pillbox to her husband's first swearing-in, so did Quayle by wearing one yesterday (which could result in the obvious crack: "You're no Jackie Kennedy"). Her swearing-in outfit, a royal blue three-piece suit, typified the political-wife look -- but at other events she broke from the mold. She shied from conservative New York designers such as Blass and Scaasi, choosing outfits designed by students and lesser known, almost offbeat design houses. For last night's balls Quayle chose a design by Omaha native Denise Ervin, a student at the Fashion Institute of Design and Merchandising in Los Angeles. It was a purple gown, pleated at the waist with a diaphanous lace top accented by rhinestones. She wore a purple bow in her pulled-back hair. Her other two evening dresses this week were by Akira, a New York-based designer known for fairly bold looks. To the dinners Wednesday, Quayle wore a black and white sleeveless silk chiffon gown with a crisscross pleated bodice and a contrasting black and white paneled skirt -- 28 yards of chiffon. Ordered from the Akira collection, the dress retails for $1,400. Her gala dress was just as complicated. It was made of iridescent slate-blue silk taffeta. It had an off-the-shoulder portrait neckline, two fabric flowers at the center, and a dropped-waist bodice of horizontal pleating. It retails for $1,200.