"President Reagan must look both modern and serious" but also elegant, says Giorgio Armani as he proposed a costume for Reagan to wear reviewing the troops at the Yorktown celebration (or any other, for that matter) on horseback.

"Blue looks fabulous on everyone," he begins, passing up the traditional Army drab colors in favor of red, white and blue. The navy blue jacket with gold buttons and leather and gold epaulets and trim on the sleeve and collar, is teamed up with a red vest and white neck scarf. A red silk scarf might be worn across the chest, and the look is completed with navy jodhpurs and dark-brown riding boots.

Armani had trouble choosing a cap. "Everything we tried looked like a chauffeur's cap," he said.

Armani has just added padded jeans to his emporio shop collection. "A person who wears jeans should be able to wear them year-round," says Armani. His basic jean is the classic denim. "Jeans are like tuxedos. When something works very well, you shouldn't change it. Young people might look well in another kind of evening tailleur, or shirt. But a young boy really looks wonderful dressed in his father's tuxedo."

The monkey is the new animal in the Krizia zoo. And if it goes like her animal sweaters of the past, such as the lion and the elephant, monkeys will be the popular motif for everyone this fall.

Mariucca Mandelli, the designer for Krizia, says that beyond the monkeys, which she chose because they are ironic and amusing, the overall shape of her new collection comes from Calder. "I've always admired him, collected books on him. And recently in Paris I saw an exhibition of Calder which gave me the idea for rounding the shape of the clothes. Even some of the pants are rounded, suggesting the shape of the top of jodhpurs.

They call him Calvin "Klane," but whatever they call him, many Milanese women are seeking out the new Calvin Klein boutique on the via Sant 'Andrea in the most chic shopping quarter in Milan. The shelves of the shop are full, and the shop owner, Marcello Rubinacci, who has several French designer boutiques on the same street, admits that Italian women are experimenting with different ways to wear the clothes. "Women in Milan prefer to match the colors more than boldly mix them," says the store's manager. "And Italian women, shorter than the Americans, want some of the skirts shortened." Prices in the boutique are about the same for Gianni Versace and Armani, which is 20 percent above the Washington price. Price hasn't curbed the business on Calvin Klein jeans. The store keeps a list of paid customers for the jeans, which sell out the minute they arrive in stock. The men's jeans sell for $65, the women's for $75 and $85, depending on fabric.

Before her marriage, Lady Diana, the princess of Wales, tested the designs of Gianni Versace with a few blouses. Last week she went all out, buying 12 items, including suede pants, a searling coat and a corduroy cape by Versace.

The strongest theme at Versace for spring is art deco, which shows up in patterns and cut of clothes as well as some extraordinary beading. "Art deco is the last style that had a good influence on art," says Versace, who has just finished restoring an art deco plaza on Lake Como.

Versace deserves credit for starting the olive drab color which now covers the streets of Milan and shows up in Paris as well. He has used it for several seasons in wool and silk as well as leather and suede. But for spring, he is partial to far brighter colors, particularly orange and turquoise.

"I'm bored with loden," he says. If women and men have not picked up on the jodhpur shape he has shown for years, he says, "give them time. It took 10 years for the loden color to catch on. I only started showing jodhpurs five years ago." For spring, he shows a short version of jodhpurs with other pants lengths and shapes. But his favorite pant now is the jumpsuit. "It is one piece and easy. For me, the jumpsuit is today."

Ruth Rabb, wife of the American ambassador to Italy, was on the Via Borgospesso last week, ordering clothes from designer Laura Biagiotti. "It makes me feel I am on the right track when I can sell my clothes to Consuelo Crespi (the socialite fashion editor) and to Mrs. Rabb, a diplomat's wife," commented Biagotti. When she started her own fashion business in Rome almost 10 years ago, she was working for a manufacturer who made dresses for the rich and beautiful. "Nothing was appropriate for me to go to work," said the designer, who now concentrates on clothes suitable for working women. "You don't have to wear a blazer if you are a company official. You can wear white linen and do your job in a feminine but just-as-efficient way."

White linen, enhanced with some of the fragile details once saved for lingerie, is a trademark of Biagiotti -- and so is cashmere. When she first designed cashmere sweaters for men, she named the company MacPhersons, even though the sweaters were created in Italy. "No one would buy cashmere unless it was thought to be British," laughs Biogotti, who has become known as the cashmere queen. She was wearing a white cashmere jumpsuit that she says makes her feel warm and healthy. "Cashmere is a pet in my life that I love and I take care of in a special way," she says. She washes it in cool water with a neutral soap and always places it in a dark place to dry. "I don't know why, but cashmere lasts longer and better when it dries without light," she says.

Her penchant for white cashmere and linen will carry over to her new perfume, made from white flowers.

The bright tomato red that is so predominant in the stylish shopping area of Milan is not a tilt to our first lady but simply a way to perk up the loden green that is everywhere. Other street favorites are bermuda shorts and knickers. The biggest surprise is to see the amount of leather and suede being worn -- not just by the young, but leather skirts and suede suits on mature women and black leather pants on all ages and sexes.

Far less apparent than six months ago are the metallic leathers. There are still bronze shoes being worn, but the metallic is not as noticeable -- maybe because the eye has become accustomed to it. Red looks far fresher, not only for shoes but for hose, belts, bags and sweaters. Even a few men have taken to red as sweaters worn under sportcoats. No damnyankees-red socks on the men, yet, or at least none that can be seen.