The gray was too light (the unpainted areas look white), so I want to cover up that brightness with paint. I usually want the color I start with to influence what goes over it. For example I never paint with pure black (it puts a 'hole' in the canvas) so I want a rich blue to be part of the 'black' in the Polly pump. And to make the pump colors rich, I under paint with the complementary colors.
Then I start adding the top colors. If you see the mottled look on the red pump, that's the green showing through, giving the paint some depth. You can really see what the green under painting does on the fire truck on my toy painting (non-petro) the pump has a thicker layer of reds over it and the under color is more subtle. In other areas I mixed colors wet-on-wet.
The 'sky' is dark as I originally thought I would do a night scene, but changed it in the final painting