Spring fashions for working women

What single fashion item would you suggest to help the working woman to update her wardrobe for spring? That's the question posed to several top American designers.

The ideas they offered ranged from a one-piece dress to a man-tailored vest and a pair of secondhand Marilyn Monroe-style, pointed-toe shoes.

Halston: ''A one-piece dress is the first thing that comes to mind. It can go to business and out. And it can be dressed up or down. Get a becoming color and style - depending on your work and social life. A one-piece dress of a simple nature can be accessorized many different ways.

''There's a trend toward simple apparel. High style is expensive. Many of the dresses featured in fashion magazines cost $500 and up. The problem with high style is that it's outdated quickly. By and large, separates are a good idea, too. They can be changed around.''

Bill Blass: ''I'd recommend a suit. It fits so well for the professional woman. For spring, we're doing suits with vests, not blouses. We decided on man-tailored vests. If a woman already has a suit, it's new to put a vest under the jacket. Wear it in a more architectural way. Find a vest or two and wear them with the suits in your closet. A vest isn't any more bare than wearing a sleeveless sweater or blouse.

''The skirts that go with the suits are straight and narrow. They're done with a slit or two for walking. For the woman who is wondering about length, I think it's important to mention that there's no one length. My advice has always been to wear what feels right for you. A lot depends on the shoe and heel height. Also, I feel that those 'dressing for business' formulas are foolish. If you look attractive in a bright color, wear it!''

Betty Hanson: ''My answer is a blouse. The working woman, for the most part, is behind a desk, so the newness comes from the waist up. While she may be watching her fashion dollar, I think we've educated her correctly. She realizes that what she buys is going to work well with what she owns. She can update with a blouse that's done in a new fabric, color, or neckline. For example, a pastel linen that she can wear with her skirts. When we go out to stores across the country, we find that women will buy one costume and then two or three blouses.

''For those who like jacket dressing, there are the new cropped ones. They're done in exciting, vivid colors and can be worn with skirts, pants, or even over dresses. Then there are vests. I've been wearing mine all week. They're comfortable to work in, and not restricting. They give such a finished look to an outfit, and they're not expensive.''

Albert Nipon: ''We feel an executive dress is the answer. Dresses are stronger than ever this season - especially the feminine-looking ones. I can't understand women who dress like men. After all, a woman's greatest asset is that she is a woman. Why dress like a man?

''Another suggestion is the executive blouse - one with ruffles or interesting shoulders. These are feminine, too. They can be worn with the skirts that are already in the wardrobe. Isn't that what fashion is all about? Adding to what we own?''

Fiandaca: ''How about a button-down skirt? There are new versions that either button down the front or at the side. They can be worn with spring sweaters and a new wide belt. In fact, you can add this same belt to almost anything to update it. You might also consider taking a cardigan or one of your V-neck sweaters, and adding shoulder pads. It's a very '83 look.

''A pair of pale hose for spring is a must, to keep the leg as light as possible. It might even be fun to go to a thrift shop and buy a pair of those pointed-toe, Marilyn Monroe-style shoes from the '50s. Some of them have open toes - just like the ones being shown in the stores.''

Anne Klein (Donna Karan, designer): ''Seasonless dressing is where it's at. If a woman invests in a couple of modern, sculptured pieces, they'll stand out on their own or look wonderful when pulled together. You might say that they're separates transcended to a heightened sense of elegance.

''As far as fabric is concerned, cashmere plays an important role in creating those new separates - whether we're talking about a long cardigan and skirt or a tank dress. They are pieces that are timeless. For those who like suits, there are the tattersalls that can be worn together or with a black blazer.

''Accessories are very exciting for spring. They add a polished edge to the clothes. Watch for the cinched belt in either elastic or patent. There are big black felt brim hats. And the perfect suit shoe is the sling-back pump. It's going to be a wonderful spring.''