Sure, 3-D printable plastics are available in a rainbow of colors. And yes, there are corn-based plastics, glow-in-the dark plastics, and even color-changing plastics with glitter mixed in. But if an engineer needs to prototype the sole of a sneaker, or a jeweler wants to produce a broach out of 18 karat gold, they're forced to turn to machines that cost more than a Mercedes.