It is back to the history books if you care to comprehend what the Paris fashion designers are up to during this week of fall couture showings. Like the ready-to-wear collections that preceded these shows by three months, there is a heavy dose of the 1940s in the fall designs with broadshouldered suits with fitted bodies tightly nipped waistlines and peplums plus a heavy injection of the early 1900s and Paris under the spell of Serge Diaghiley and the Ballet Russe, for wich Picasso designed some costumes. The bright colors used by Picasso, particularly in his pink and blue period, show up in color blocks in the new Saint Laurent collection along with Picasso-like faces painted on some of the clothes. And from Picasso's Spanish origins, there are matador-like suits and flamenco dresses. To complete the cycle, Paloma Picasso, the artist's daughter, was applauding the collection from her place in the front row.

Saint Laurent is not alone in his fascination with Diaghilev. Jules-Francois Crahay has also made a salute to Diaghiley with his brocaded tunics, harem skirts and Oriental colors.

Sure to spin-off from the current shows is the hourglass shape at Christian Dior and Givenchy, with broadshouldered jackets with set-in sleeves with fullness at the top, and tiny waists, balanced by full skirts that replace the pencil-slim variety currently popular.

Peplum suits are a favorite theme for day and show up in the collections at Dior, Lanvin and Ungaro among others.Bright colors and black are important on the palette, and textured fabrics such as velvet and tweed show up frequently.

The Paris couture has clinched the question of hemlines. They consistently sanction shorter hemlines than many of the clothes women have in their closets. The popular Paris hemline for day hovers just below the knee. Chanel's formula of hems that stop at the tip of the finger when the hand placed over the knee is a popular length. CAPTION: Picture 1, Yves Saint Laurent's "Picasso suit" for fall; AP ; Picture 2, A bull fighting outfit from Picasso's Spain; UPI