Lovely picture

. You got enough layers IMO, though. As I interpret the picture there are already six layers.
As english ist not my native language I'm not good at explaining what I mean. I think the problem is your choice of using the colors. For example, you use the yellowish color for highlights or the water (I simply guess it's water

, I mean the yellow bar) in the foreground layers, but also for fading the building in the background. Then again you use the same color for the planet. They seem to be on one plane.
The colors definately should help separating the different layers. You should use the colors (more) exclusively on the different layers. As you want to keep the color count low I'd recommend reducing the number of layers.
On a side-note: The red sticks out too much. But that's just personal taste.