FIRST LADY Rosalynn Carter and National Gallery of Art curator David Rust have someone in common: Mark Hampton.

Hampton, it sometimes seems, is in half the photographs in the fancy shelter magazines these days: Hampton with family and Christmas packages; Hampton's Christmas tree (decorated in July to suit House Beautiful's deadlines); Hampton holding forth on decorating in Sunday magazines.

Hampton has also appeared recently at the White House, decorating it for Christmas. And Hampton is the featured speaker at a brunch and lecture at the Washington Antique Show Jan. 15 at the Shoreham Americana.

And, though, you wouldn't hear it from his (his services seem as confidential as the confessional) he has been engaged recently to help with the interior design of the Georgetown home of National Gallery Director and Mrs. J. Carter Brown. With Brown's well-known and exquisite taste, the job must seem like carrying the proverbial coals to Newcastle.

In his speech at the antique show, Hampton plans to tell about the clients he flies to in Louisiana or the ones in Ft. Worth.

Hampton specializes in a rather rare group of clients - those who not only have the money to buy whatever their hearts desire but also the taste to say what they want. One Hampton client, for instance, goes to London to buy fine English furniture. Another has a large collection of rugs. A third collects French furniture.

They're not the sort of people, to put it mildly, who call a decorator and say, "Please come and do my house," expecting the decorator to arrive with a truckload of furnishings down to the last ashtray and Boehm bird.

Hampton's forte is to provide professional consultation for people who, as one person put it, don't buy furniture but have furniture and who add to their collections with great pleasure and personal expertise.

"When you come into a house I've decorated, I wouldn't want you to say, 'Oh, what a great Mark Hampton room.' Instead, you should recognize and appreciate the taste of the owner. The most successful decorating jobs are always the most personal ones," Hampton said in an interview the other day, in between the holly and the poinsettias at the White House.

Two good cases in point are the way Hampton worked with the White House and with David Rust. In the White House, Hampton's plan "was to please Mrs. Carter and not get in the way of all that grand architecture. Mrs. Carter was fabulous to work with. She knew what she wanted. She approved my first plans and then stuck to them. She didn't fuss around with the design while we were putting it up. I disapprove of those who want to change things for no reason."

Hampton used natural greenery and the poinsettia trees in the great hall, without ornamentation, so that the tree in the Blue Room with its 1,500 odd decorations would surprise the eye. The effect was very handsome, but very natural: understated, not at all the overdone decoration often seen in past White House Christmases.

Hampton may be coming into his own on the national scene. But Rust first used his talents nine or so years ago when he was putting together his Georgetown house. Rust, the National Gallery's curator of French painting, has a personal collection of art said to be second locally only to the great collection of David Lloyd Kreeger. Rust has a number of magnificent 17th-century Italian paintings, several masterpieces of pre-Raphaelite painting (both a Dante Gabriel Rossetti and a Sir Edward Burne-Jones) and a fine collection of drawings, not to mention many contemporary works, including a large Sam Gilliam painting over his king-sized bed.

"Obviously in David's house, the art is the thing," Hampton said. "So everything has to be subordinate to that."

Rust first hired architect Hugh Newell Jacobsen - Washington's own shelter magazine star - his Christmas tree was in HOuse and Garden magazine this year. Jacobsen designed a fine glass wall, with doors on either side, to look at Rust's south garden. Jacobsen's real triumph is what is said to have been one of the first black swimming pools in town. The long, skinny pool is encircled by a stone patio that forms the pool's coping. The pool's blackness makes it look like a reflecting pond; it's narrowed slightly at the end to fool the eye and make it seem longer than its 12 by 30 feet. It is one of the few pools that looks good in December as well as July.

Jacobsen also added a large glass area and a skylight to Rust's bathroom, to turn it into a wonderfully indulgent greenhouse/solarium, the black cat's favorite room.

Hampton, in turn, egged Rust on to paint the living room persimmon, textured and striated with a lacquer finish. "I wanted to paint it this red - it's the same color that you find in a great many paintings in the room," said Rust. "But Hampton gave me the courage to go ahead."

The walls in Rust's dining room were covered with a dark green burlap, another fine foil for the paintings.

"In these Georgetown houses," Hampton said, "I think it's wrong to paint everything white. There's not much light in them anyway, even when they're opened up toward the south. So the trick is to give the walls some color, to keep them from seeming dark and gloomy.

"I use the same idea in Manhattan, especially in the dining rooms, which tend to have very few windows. You need all the color and mirrors you can put in them."

Hampton is not one who thinks that small rooms can be made to look larger by painting them white. "That's one of those silly things that the 'experts' say. White makes a room look smaller. In my last two apartments I've had small entry halls. Painting an entry black is just right. You can't really tell where the walls are; they just vanish. It makes the area seem much larger. Black is easy to live with and it lends a wonderful atmosphere."

Hampton's own living room is red - lipstick red. "I have a great many different shades of red objects. I especially like Japanese lacquer work, but they all fit in together well. Red is a good neutral background. It is especially good for 19th-century paintings. All the galleries back then had dark red or blue backgrounds."

The green burlap in Rust's house adds texture - light and shadow - to the color. It has another practical purpose. Like the carpeting on the walls of collector David Lloyd Kreeger's house, it makes it easier to move paintings. Rust, like most collectors, adds paintings, so everything has to be moved to make room for the new addition. On painted walls, removing an object leaves a ghost image.

The curtains in both rooms are simple, nonassertive - stripes in the dining room, geometrics in the living room. "You have to face the fact that nothing should try to beat out the pictures," Hampton said.

Hanging drawings on a mirrored wall as Rust's idea. It works well because the mirror provides a neutral space, keeping the pictures from seeming to be too much. Rust had another grand idea that he looked to Hampton to execute.

"When I was in London a year or so ago I found a very handsome Persian rug saddle bag. I asked Hampton to have it made into a chair for me. It took a year, but it was worth waiting for."

The chair, a pleasant Victorian conceit, now lives happily at one end of the living room. Rust, like all collectors, sometimes finds he has bought more than he intended. He was looking in London for two Regency chairs to go by the fireplace, and he found them all right - attached to four matching ones. At his New Year's dinner for 30-odd, he found them very serviceable, scattered around the tables for four set up in the study alcove of the living room.

Rust has other fine antiques: a chair by the French master furniture family of Jacob and a chest by Andre Charles Boulle, a French ebonist of the Louis XIV era.

"To show off that furniture, we deliberately kept the other furniture low key," said Hampton. "The straw sofa and the soft chairs are comfortable but not assertive. I think Georgetown houses are too small for the great modern masterpieces such as the Barcelona chair."

Rust also chose the handsome Tunisian rugs in the living room and his daughter's bedroom. The rugs look as though they were made by needlepoint, so intricate are the pictures. Upstairs in his daugher's room is a real work of needlepoint art - made by Rust's mother, who also did the handsome geometric pillows scattered about.

Hampton thinks everybody should decorate the house with personal objects. "I think it is dreadful for people to get rid of things they love just because the thing is not the creme de la creme currently. A lot of people confuse having a decorator with giving up their own taste. You shouldn't do it. Your taste may be more interesting than the decorator's. Besides, I think we've moving into a period - call it a Victorian revival if you like - when people appreciate a cozy, homey atmosphere, with lots of green plants.

"But by all this, I don't mean we should all have a lot of mindless clutter, nor all the family snapshots in the living room or sick-looking palms strewn about. Anything can work - but it all has to be done right."

Hampton speaks at the Washington Antiques Show at the brunch/lecture at 11 a.m. Jan. 15. The Washington Antiques Show, the biggest such event in the area during the year, will be Jan. 11-15 at the Shoreham Americana. The 44 antique dealers from all over the East Coast will show and sell their wares from noon to 10 p.m. daily and to 6 p.m. Sunday. Ticket information is available from Carol (Mrs. Robert) Foley, brunch chairman, at 299-4681. Admission to the show only is $3 at the door.

David Sanctuary Howard of London, who has just written a book on Chinese armorial porcelain, will speak on that subject at a luncheon at 11 a.m. Thursday. Clement E. Conger, White House curator, will speak at Young Collectors Night, 6:30 p.m. Saturday. The show includes an exhibit of the Chinese Export Porcelains from the collection of Mr. and Mrs. Euchlin D. Reeves, lent by Washington and Lee University. Verbal appraisals ($5 for the first item, $2.50 each additional) on antiques will be offered every day 10 a.m. to noon: Wednesday, silver; Thursday, jewelry; Friday porcelain; Saturday, prints and paintings. More information is available from the Antiques Show chairman Jane (Mrs. Roberts) DeGraff, 265-1259.

The event annually benefits the Thrift Shop Charities: Child Health Center Board of Children's Hospital, Children's Hospital National Medical Center, Prenatal Clinic of Columbia Hospital for Women, Hospital for Sick Children and St. John's Child Development Center.