For highlights I usually shift my hue towards yellow and up the saturation, and for darker shades I shift it towards darker purple/blue's and lower the saturation, depending on the "base color". It's mostly eye-balled, I have grown more pleased with high contrast from shade (often black) to highlight.
This is just absurdly awesome. Just how many million years did you spend to perfect your craft like so!? If i could fave something more than once, this would be one of the things i'd never get done fave'ing. WELL DONE.