Amid the sea of black and gray that is New York in December, you might be inclined to flash a bit of Technicolor for a change. On Monday, ToyWatch, the Italian challenger to the plastic-timepiece throne of Swatch, will unveil its first U.S. store (509 Madison Ave., nr. 53rd St.; no phone yet), selling over 160 styles, from the fluorescent watches seen here ($175) to a small-batch, silk-band collaboration with fashion label Missoni.