A/B test them. Pick something arbitrary that looks pretty good for the initial version (like the initial top-ads color for Google's May 2010 websearch redesign was "Cosmic Latte", the average color of the universe). Make it configurable in the source code, either through template variables or frameworks like Sass. Then vary one parameter at a time and measure your business metrics, and do what the data tells you to do.

It's important for designers to get the big-picture stuff right, like page layout and interaction model. For pixel-perfectness, just do what the data tells you.