Those must be the scarlet and white silks of the CBS-TV stable that Kristy McNichol, 17, is wearing as she sits astride a big mount named Gilford. The tomboy of the Family sitcom series stars this week in My Old Man, a TV movie in which she is Jo Butler, the track-wise daughter of a down-on-his-luck horse trainer, played by Warren Gates. The film is out of a short story of the same title by Ernest Hemingway, but the bloodline is a little thin. Joe Butler, the American boy in Hemingway's tale about seedy racing in Europe between the wars, never got to ride Gilford. McNichol does, and if you want to know how she fares, tune in.

Turning 83, famed American Composer-Critic Virgil Thomson proved to be as vivace as the music he has written for every mode from concert hall to films over half a century. Tendered a birthday party at Brentano's bookstore in Manhattan, Thomson ignored the limousine that had been sent to fetch him from his apartment in the fin de siecle Chelsea Hotel and marched to the festivities on his own. He also chose stairs instead of an elevator and a hard chair rather than a soft one, but he did consent to pose at the piano with his cake and a group of fellow musicians that included Conductor Andre Kostelanetz and

Pianist Bobby Short. That did not mean that the feisty composer had been defused. When one admirer became too mushy, the balding Thomson protested: "Get this man out of my hair."

"This time," sighed a friend, "Christina's caprice has cost her $10 million." That presumably includes the tanker and the London flat that Christina Onassis, 29, Greek shipping heiress and stepdaughter of Jacqueline Onassis, has turned over to her estranged third husband, former Soviet Maritime Executive Sergei Kauzov, by way of closing the books on an unhappy 15-month marriage. She hated their Moscow apartment even though Kauzov, as a worker and husband of a notable foreign person, was allowed more space than most Muscovites. He was discomfited by her idle pleasures, including those lazy, sunny lunches on Skorpios. Said one of her chums: "How could he, for instance, accept eating under a parasol held for him by a servant dressed all in white?" Christina's whirl is now Manhattan, where she went discoing at Studio 54 last week with Nikos Boukis, a childhood friend whose family is also into ships.

There are some moves that the Soviet Union's World Champion Chessmaster Anatoly Karpov, 28, would probably prefer no one kept track of, including his wedding five months ago to fetching Irina Kuimova, 25. Certainly TASS chose not to. Announcing the birth in Moscow of a son to the Karpovs, the newspaper recalled only that the couple had been married "this year."

On the Record

Thomas Monetti, Waldorf-Astoria catering executive: "People eat more mousse in crisis situations. Maybe it's easier to spoon up than pie or cake."

Joao Baptista Figueiredo, hippophile President of Brazil, on the oil crisis: "The only solution is to tighten belts, walk and keep a horse in your corral."

Natalie Wood, actress (Meteor), on critics: "Anyone who says it doesn't hurt when they zap you is not to be believed."

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