Once again it's time for the bright colors, lovely perfumes, tangy refreshment and soothing natural sweetness of fresh fruit sherbet. Le sorbet (sherbet's fancier French name) is the dessert rage for everyone who doesn't eat dessert.

Who makes the greatest sherbets in the world? My vote - and I suspect, the vote of virtually everyone in Paris - goes to an obstinate, secretive, taciturn man who runs a tiny cafe des sorbets on an island in the middle of the River Seine in Paris.

If you walk around the outside of the Cathedral of Notre Dame to the back of the church, you come to a small footbridge that leads to the Isle Saint Louis, where the houses are old and the streets cobbled and narrow.One of the main cross streets is the Rue Saint Louis en I'Isle, and there, at number 31, is Berthillon's.

When I discovered him at least 15 years ago, Monsieur Berthillon owned a neighborhood sherbet parlor, virtually unknown beyond the two or three surrounding blocks. Today, he is a celebrity - but give him credit for having steadily refused to leave or enlarge his miniature cafe and for turning down all the gold-plated contract offers dangled before him. His one, concentrated passion is to work in the small "laboratory" behind his shop, inventing and making sorbets.

Some months ago I decided to talk to Monsieur Berthillon to learn some of his secrets. I went to his cafe and asked the waitress to give my card with the request that I might visit with him for a few minutes. The message came back that Monsieur Berthillon always was extremely busy, and what did I want to see him about?

The waitress carried the negotiations back and forth. They were considerably more complicated than the SALT talks between the United States and the Soviet Union. Finally, he agreed to se me if I would come at 6 p.m. the day after next.

I was there exactly on time, but Monsieur Berthillon kept me waiting for two hours. I was furious, of course, but my rage was somewhat soothed by the fact that, during the two hours, I was able to taste 27 flavors of superb sorbet.

When I did at last get into the back room, it had been cleaned up. All the equipment had been put away or covered so no one could even read the nameplates on the machines.All the ingredients and tools were in closed closets and drawers.

Monsieur sat behind his desk. He did not invite me to sit down, but I sat down anyway, I said it was a great pleasure to talk to him. He said there was nothing really to talk about. He warned me that he never answered questions; how he did his work was his business.

I kept my cool and made pleasant conversation all around the subject. This way I learned a fair amount about his general principles - no secrets, just general principles. But our talk ended rather suddenly under chaotic circumstances.

I had just been saying how much one was aware, while tasting Monsieur Berthillon's sorbets, of the extraordinary natural freshness of his fruits.

The man smiled and said that, of course, he would never dream of permitting anything but the freshes fruit to enter his establishment.

At that precise moment, a young girl in a white coat walked in carrying an industrial-size, 10-pount can of American White Rose yellow cling peaches.She crossed the room to the spot where a can opener was screwed to the wall and proceeded to open the can of peaches. Monsieur Berthillon began to scold her - in Italian. (He had been speaking to me in perfect French.)

I made a great mistake. I said, "Parlo italiano, signore" (I speak Italian). His face flushed with rage. He stood up, glared at me, pointed to the door and almost yelled, "Vatenne!" (which is, more or less, Italian slang for "Get the hell out of here").

What I have learned from talking to Monsieur Berthillon and from tasting his magnificent sorbets has enabled me to reproduce them satisfactorily in my own kitchen. And I certainly do not mean the average, ordinary kind of home sherbet, with fruit that has oxidized and discolored, with perfume that has vanished into thin air, and with ice crystals that crackle between your teeth. I mean a sherbet that looks and tastes professional, that could be the party dessert at any formal dinner.

Sherbet must have the natural scent and sweetness of the perfectly ripe fruit, with velvety smoothness, yet with a delicately light chewiness. You must, of course, have a freezer that runs very cold, down to zero Fahrenheit degrees.

It also helps to have an efficient, modern, electric freezing machine. The one I find consistently best is the Salton which works inside the freezer of my refrigerator. It does not have to bonded to the floor of the freezer because it has fan that draws in the ice-cold air and circulates it around the canister while the dashers are turning.This avoids the formation of ice crystals. Fresh, ripe fruit must be frozen as quickly as possible to avoid oxidazation and discoloring, with quick loss of flavor and perfume.

I wish I could claim that the following recipe was given to me by Monsieur Berthillon. He does not hand out recipes. But this sorbet of lime juice and rum is the style of one that would be served in the Berthillon cafe.

Use one of the great "Plantation" rums from the French island of Martinique. Join it to the juice of fresh, just-squeezed limes and the zesty oil of their skins. Then you and your dinner guest will have the distinct feeling that you are on the Isle Saint Louis in the heart of Paris.

SORBET OF LIMES WITH RUM IN THE STYLE OF MONSIEUR BERTHILLON (6 to 8 servings) 1 pound (2 cups) superfine sugar

Limes, up to 10 fairly large, fresh, ripe ones (enough to get 1 cup of juice).

1/2 cup French Martinique plantation rum

Kitchen equipment: Stainless steel, 2-quart, lidded saucepan; cutting board and sharp knives; wooden spoons; sharp potato peeler or zester for skinning limes; juice extractor; rotary ice cream machine; 2-quart lidded freezer jar.

Average time required: Sherbet-making is mostly waiting time. Whil you don't have to be in the kitchen, always make it the day before, so it can mature and ripen overnight. Allow about 30 minutes for infusing and simmering the limes skins to draw out the oils, then about 2 to 3 hours for the freezing. Store it overnight.

ACT 1 - The lime-sugar mix: Put into the saucepan 3 1/3 cups of water and heat to boiling. Meanwhile, choose the 3 limes with the best skins and, using either a potato peeler of a zester, remove then pices of skin from the limes and coarsely chop the peelings. As soon as the water boils, stir in the sugar and the chopped lime skins. Keep it all bubbling and stir until the sugar is dissolved; then simmer gently. Cover the simmering mixture for 30 minutes to let the lime skins infuse their flavor oils into the sugar syrup.

Meanwhile, halve and squeeze the juice from the 3 limes you have skinned. Use additional limes until you have 1 cup of juice. This usually requires about 8 limes, more if they are a bit dry. Hold this juice, covered, in the refrigerator.

You will save time if all your containers, tools, and utensils are ice cold before they are needed. Chill your quart storage jar and the canister of your machine (including, of course the dasher) in the freezer.

When the infusion is completed, turn off the heat, let the syrup cool to 90 degrees then pour it into the chilled 2-quart jar. Cool the syrup rapidly in the freezer for about 15 minutes, until it feels just cold to the tip of your finger. Now stir it into the lime juice sieve (preferably of nylon or stainless steel, to avoid metallic taste) to get rid of the bits of skin. Your mix is now ready for freezing.

ACT II - Freezing and ripening: I pour the mix into the ice-cold canister of the freezing machine, fit the dasher in place, put on the lid, and attach the canister to the motor in the fram of the machine. It stands on its three-legged base in my freezer (the flat electric cord comes out throught the freezer door) is plugged in, and the motor switches itself off automatically when the sherbet is frozen. Depending on the type of mix, this may take anywhere from 1 1/4 hours to 3 hours. (If you will know how to make your own arrangements.)

As soon as the motor stops - before the sherbet has become very hard - I check it for ice crystals. Usualy there are none. But, if the sugar mix is wrong and too much water has caused crystals to form, I quickly run the entire sherbet for a few seconds through my food processor to homogenize it, then return it instantly to the freezer and let it harden completely. Overnight, tightly lidded, it will develop flavor and perfume.

ACT III - Softening and serving: The next day the sherbet may be so hard as to almost need a chisel and hammer to break it up. You must deliberately soften it, but not too much. As my dinner guests arrive, I move the container with the sherbet from the freezer to the refrigerator. At the same time, I put my sherbet glasses into the freezer; they must be absolutely ice cold. Also, set the 1/2 cup of rum, covered in the refrigerator.

As a main course is served, check the sherbet.It should now be just soft enough so that 3 tablespoons of the rum can be quickly and lightly stirred into it. If this action softens it too muct, return it to the freezer for 15 minutes to reharden.

Finally, when the softness is exactly right, serve it in the cold glasses and make a small depression in the top of each portion. Into this little hollow pour an extra teaspoon of so of the rum. Serve instantly.

Working notes: It is always best to make your sherbet the day before you need it. It actually improves when kept overnight in the freezer. If you make it a few hours before you dinner part, you can't be certain the sherbet will be properly frozen in time to be served.

Incidentally, this recipe represents a basic technique you can adapt to other citrust or pureed fruits. You can also infuse other aromatic herbs or spices into the syrup - cinnamon, cloves, fresh peppermint leaves, etc. How strong you make the flavor depends on how long you simmer the aromatic ingredient.

Menu notes: A few days ago, we served this lime and rum sherbet as part of a small dinner party menu, which was easy to prepar, yet appreciated by the g ests. We began with soup of pureed young carrots - very sweet, with a good balance of herbs. Then, for the main course, a light, butter-and-cream version of a blanquette of veal, accompanied by tiny spring peas. We ended with the sorbet and coffee. CAPTION: Illustration 1, no caption, By Robert Barkin - The Washington Post; Illustration 2, no caption