I'm constantly dicking around with levels, colour balance, vibrance, curves, etc. It's fun, and, certainly quick and useful. But I think, actually, what people are asking when they question how Helm arrives at his colour choices isn't the subtle tones over-arching the palette, but his generally surprising choice of hues in a small area. The branch is made up of orange, mustards and green with vibrant purple shadows, for example.
I often find I use more interesting colour combinations this way when I'm using a limited palette. But best when it's a sort of generic one, for various scenes/objects. If the palette was tailored specifically for that one scene/item, I guess I'm afforded the option of being more conservative.