January 2, 1986

THE clients, who had just bought an apartment in Trump Tower, asked their designer, Allen Minars, to do something that was, perhaps, a little different; maybe something with some color. In retrospect, looking around their elegant but startling living room, they admit they hadn't really known exactly what the result would be. Mr. Minars took his clients at their word, and rather than shopping in showrooms for predictable pieces, he decided to buy art furniture. The resulting apartment summarizes one of the latest and strongest trends in New York design, 1986: the chairs, tables, desks, cabinets and lights are all designed by artists as one-of-a-kind, signed pieces, with strident colors, odd shapes and angular profiles. Despite their individuality, they add up to a coherent look and a strong statement.

The stock market Tuesday closed out the most profitable year ever for investors, sending them into the new year with a renewed sense of confidence that bodes well for economic expansion in the months ahead. As blue chips led the way, the value of the universe of common stocks rose by $462 billion last year, with most of the gain sweeping through the market over the past three months, according to Wilshire Associates of Santa Monica, Calif. This easily topped the previous best gain of $303 billion in 1980, although the percentage increases of the popular market indexes did not set records. Most important, the windfall gains in stock prices should give added stimulus to the economy because it increases household wealth and reflects expectations of brighter business conditions, thus encourging corporate and consumer spending. At the same time, consumers actually have more money in their pockets because the recent rally has largely been institutionally inspired as individual investors continued to be net sellers on average.

After eight years of arduous, often frustrating, negotiations, Spain and Portugal became members of the European Economic Community today. Their inclusion as the 11th and 12th members was automatic after the signing of a treaty last June and its ratification since then by their Parliaments and those of the other Community members. The expanded European Common Market encompasses 320 million people, one-third more than the population of the United States. Spain and Portugal will also take up places in the European Parliament and other Community institutions.

When people on Roosevelt Island define their community, they don't exactly make it sound like the quintessential New York neighborhood. ''Everyone knows everyone else's business,'' said Hattie-Jo Mullins, a mother of three. ''It's like Peyton Island.'' ''It's like living in the little house on the prairie,'' said Mrs. Mullins's 16-year-old daughter, Lesley.

Edward Irving Koch took the oath of office yesterday for his third term as Mayor of New York City, then pledged that despite a ''Federal assault'' on the city's budget he would work to resolve the problems that still elude solution. At a good-humored ceremony on the steps of City Hall, the old building's columns standing as his backdrop, the 61-year-old Mr. Koch spoke of the city's housing crisis and its homeless, its schools and its infirm. Promising he would ''never be satisfied until these problems are solved,'' he said, ''There is no challenge too great to confront and respond to.'' But even as he looked toward the problems ahead, the Mayor looked back at his successes. From the beginning of his speech to its conclusion 20 minutes later, he presented a dual message: ''a firm sense of pride'' in his record and concessions that difficulties remain. He noted his 78 percent election victory one moment, called the city's housing shortage ''our most severe and intractable problem'' the next.