The excitement in Jordan where Elizabeth (Lisa) Halaby will marry King Hussein on June 15, has practically eclipsed that other royal wedding, that of Monaco's Princess Caroline. By now, Caroline's choice of a Dior gown for her July marriage to Philippe Junot is old news. In Amman, meanwhile, there are only scattered clues as to whose gown the 26-year-old Halaby will wear for the quiet ceremony in the palace.

"Really classy" is how one Halaby intimate describes the bride-elect. A colleague at Royal Jordanian Airlines, where Halaby has been director of facilities, planning and design, calls the willowy blond "one of the really stylish ones." Halaby may show up for work one day in jeans or slacks - "very tastefully", a Diane Von Furstenburg dress or a costume mix of a loose, mannish jacket, big skirt and shirt with necktie. Sometimes Halaby sports an Ivy League look, reminiscent of her years of Princeton.

While some Halaby friends surmise she'll wear something "off the rack" for the wedding, gossip in Amman has her jetting off to Paris like so many chic Jordanian ladies. According to her mother, Laura Halaby the wedding dress design is already in the works.

Mrs. Halaby, who has been shopping for Lisa on Seventh Avenue, particularly at Halston and Oscar de la Renta, says, "we've always shopped a lot together." Her daughter, a size 8 or 10 depending on the particular make of the clothes, usually picks up things off the rack in Paris and New York "and is not too bad at making the alterations herself," says Mrs. Halaby. "She has always kept within a working girl's budget," she said.

Regardless of the designer, Halaby is expected to wear a long, white gown, as did Hussein's three previous wives. His first wife, his cousin Dina, wore the most elaborate Paris design. It also had the longest train. Wife No. 2, Princess Muna, was married in a slightly less sumptuous gown. The late Queen Alia wore a simpler gown still with a cap of fresh flowers.

Hussein himself wore a dashing military costume of black jacket and red trousers for the first two weddings, and a dark business suit for the third.

B.Y.O.P. - parasol, that is - which is what Mrs. John Sherman Cooper did last week for the luncheon Joan Braden hosted for the London designer Thea Porter. (The thoughtful hostess did provide parasols for those who had forgotten them.) "I can't remember the last time I got dressed up for a ladies' luncheon," said Barbara Howar, who nonetheless did, along with about a dozen other guests.

Porter has earned a following with her evening clothes, notably a caftan made from two saris appliqued with antique embroidery, and a slender, two-piece black dress of ribbon-encrusted chiffon. Of another Porter design, a bare-topped, silver-sequined number, barbara Howar commented ruefully. "It must be the only dress for which I'm too buxom."

Porter dates her interest in unusual fabrics to her childhood in Damascus, where her father was a missionary. She opened a shop in London in the 1960s to sell Ottoman decor, fabric and antique caftans. Her customers then included Pink Floyd and the Beatles.

It was long-time Washington customer Evangeline Bruce who got Porter started on daytime clothes this year. Another steady customer is Elizabeth Taylor Warner, who loaded up on eight Porter designs last August and still more in November. Many were in the bright colors her husband John Warner prefers. "It's easy to design for a woman like that," says Porter. "You don't think about her figure.Everything goes back to the eyes.

(Locally, Porter's clothes may be ordered through Saks/Jandel.)

More accolades for Mary McFadden, who'll be awarded her second Coty in October. The award honors McFadden's fall collection, her first to include daytime clothes. McFadden was in town to lecture on the history of fabric design as part of the Smithsonian Associates' "Extraordinary Women in Fashion" series.

In running for their first Cotys are Willi Smith, Charles Suppon, Perry Ellis and Bill Atkinson for women's designs; for men, Jean-Paul Germain, Robert Stock and Suppon. As a three-time winner, Bill Kaiserman will be elevated to the Coty Hall of Fame; special awards will go to Danskin, Joan and David Helpern and Head Sportswear.

Rose-colored reveries: Helga Orfila, wife of the OAS chief, wore oversized sunglasses with a heavy rose gradient tint shading lighter from bottom to top to a recent dinner with Paris designer Sonia Rykiel. Another guest, Henri Bendel president Geraldine Stutz immediately pounced on the idea for her store.

The perplexing question of the moment, at least from the volume of our telephone calls, is whether it is appropriate to wear a long dress to a daytime event, say an afternoon wedding? Well, very few rules exist in fashion today, but one that seems to hold fast is that long dresses should not be worn before 5 p.m. Many people still choose not to follow it, but for those seeking rules, there you are.