I know it sounds like a cheap cop-out answer to say "It depends on the picture" but it depends on the picture.
For example, when I color robots and Transformers, often it's just a lighter shade of black to go over the metal armor. But when I color people, since flesh tends not to be as stark as metal, I'll pick the color from the base color, and go from there. Pearl was picked using a swatch slightly darker than her skin color. But Amethyst, using the same color tended to over-saturate the shadows, so I actually used a slight blue-grey, which was actually LIGHTER than her skin tone, until I hit the Multiply layer.