REally nice image Squirrel,
I was about to tell you it's all about the colour in this and then I read Tolouse's comment. To me it's the compliementary colour scheme that makes this piece work well. I have made a decision today to have my colour wheel sitting next to my computer. Darn expensive to buy but invaluable once you've got it. Up until now I've used it when I've designed textile pieces. I'm a very visual person as I'm sure most people on this site would be. I think having a colour wheel nearby will help me to think the colour harmonies thru better at certain times.
Here is a little excersise that you might like to try, but I am also posting this for the benefit of others as well. Try using the digital paint to create a grey scale. White first and black at the other end, then try to obtain graduations of grey from the lightest you possibly can to the darkest you possibly can before total black. This is one thing to do it with real paint but quite another to do it digitally. You can then take this excersise further and do it with any primary or secendary colour on the wheel.