So, I took your drawing into my art program and set the saturation to 0.
Lots of your colours are midtones and there's not too much difference between the environment and the characters.
I added a few multiply, add and overlay layers over the characters quickly. It's not the best job and I'm no expert, but I think the characters are a lot more separated from the background.
Here are the new values. They're not hugely different, but I think there's a bit more differentiation of the characters from the background.
I hope you found this helpful!