I had this faint memory of some graphs goat made for the chaos engine critique (
http://www.wayofthepixel.net/pixelation/index.php?topic=1025.0 ) - that post is a pretty decent primer on hue shifting on its own. Anyway, I wanted to map out the hue and saturation of all the colours in the Indigo piece, here's what I came up with:

This is what I know about hue shifting:
Use warmer colours (reds, yellows) for highlights and cooler colours (blues, greens) for shadows. If you're colouring something green, use a yellower green for the highlight and a bluer green for the shadow. This is only really true for natural light or similar, though - if your light is green, you can use warm colours in the shadows and it'd still look right.
You can pull off larger jumps in hue whe you're using less saturated colours - grey goes with everything. With Indigo's piece, his most saturated colours (the highlights) don't shift around much - the shift to blues happens in the midtones where his colours are less saturated.
Anyway, I hope this is helpful. Generally there's not a lot of procedure to it, you just go with your gut and then fix it until it looks good.