Here's an edit:
I'd try and watch your single pixel usage, I feel like sometimes you are trying to use a lot more detail than is really attainable, I do the same thing a lot and it just muddies things up. In your image the lightest colors were definitely taking the eye away from the actual characters, same with the darks. A lot of contrast usually signals that something is in the foreground. The background I tried to reduce contrast, and you can use like complementary colors to your advantage if you want something to be close in value but still stand out like the blue and the orange highlights on the car.
I also added outlines to see how they worked because they usually help seperate things from the background.