I worked a bit differently for this and tried to imagine how a real artist makes a picture. I did a little thumbnail sketch first which I never ever do, there's never any planning stages usually and whatever first doodle I do ends up as the final image. For this one though there was two pencil stages before I started laying in the colour.
I tried to work with different colours in this piece too, sort of a warm yellow highlight, cool purple shadows and like a green mid tone. I am pretty colour-blind though so it's probably actually all in blorgentas™ and graghcillian™ hues.
I had an old sketch of a robot that I based the ones in the picture on, he pops up over on the right there. You can see a couple of photos I took for reference in one of the stages too. The idea was this was set in Wellington and the sea had reclaimed Lambton Quay and had brought a bunch of monster crabs with it. Obviously the only way to resolve this type of issue is to send in a bunch of human-shaped robots with guns and a futuristic tank.