IT'S red and yellow, nonpolluting, durable and inexpensive. In fact, the Cozy Coupe was the world's best-selling car for much of this decade, but you can't take it on the highway. Not that you'd want to: it goes only as fast as a child can walk. A young child, like 2 years old.

If children's cars sales were promoted as actively as their grown-up siblings, the flashy Coupe, made by Little Tikes, would rank among the top-five sellers in the United States (313,000 of them were sold last year; in Britain, about 100,000 were sold).

There are other plastic toy cars on the market, like the red Snuggle Bug made by Step 2 in Streetsboro, Ohio, and the TKV (''two-kid vehicle'') and Sports Treker, both made by Today's Kids, which is in Dallas.

But the Cozy Coupe wildly outsells them all. Step 2 will not say how many Snuggle Bugs, at $40 each, are sold a year. But Today's Kids says it has been selling 80,000 TKV's worldwide a year at $55 apiece, and 260,000 Trekers a year for $30 each.