Attempt more hue shifting with the shadows and highlights, to make the sprites pop out. As I mainly when coloring, I typically allow for more difference. To help with contrast. Put all of your work under a painted white layer -> color -> Turn on and off. It will show the image as black and white. If you're not confident in your color ability or just need more contrast. It helps. I work on artwork in a zoomed out perspective to make sure the outline of the character is noticeable.