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The earth didn't move on Jan. 1, 1954, when NBC aired the tournament of Roses Parade from Pasadena, California -- the first national, commercially sponsored TV program broadcast in living color. In fact, most of the viewers in the 21 cities that carried the show could only imagine the colors, since virtually all TV sets then were still black-and-white. But it was a beginning. Within 20 years, not only did nearly every American home have a television set, but most of the sets were in color. Families were eating their dinner on trays in front of the TV rather than miss one minute of a show. Madison Avenue had found a powerful new tool that allowed it to reach huge numbers of consumers right in their homes. Advertising flowered with creativity, producing its own 60-second shows called commercials, building on copy written for print (``Wonder Bread helps build strong bodies 12 ways!'') and eventually creating new art forms with jingles (``It's the real thing''). The revenues ($35 billion a year by 1994) financed a vast new wave of entertainment and information programming, from Bonanza to live coverage of presidential election campaigns. Now it's happening again. A NEW MEDIUM has arrived, and it is bringing about a change in marketing that is potentially even more profound than the coming of television. As it reaches into the tens of millions, the number of computer users is suddenly attracting advertisers' attention. During the past few months alone, thousands of companies eager to reach this burgeoning audience of upscale consumers (estimated average income: $55,000) have begun advertising online.