"Most classic color combos in design are those borrowed from nature," says California designer and color expert Kelly Berg. Case in point: yellow and green, reminiscent of sun and plants. "If nature tells us they work together, then they work together," Kelly says. And since there isn't just one yellow and one green, the combination can create many different effects. Use the two as a base, Kelly suggests, and then add a third and a fourth color to the mix, such as blue or red.