The main reason the whole piece looks flat is becuase everything is drawn that way. Reminds me of an Egyptian hieroglyph. Try drawing things with some foreshortening, overlapping and perspective. Adjusting your colors won't acomplish anything.
If these terms sound new to you, then I recommend picking up a good drawing tutorial from a class or book.There are some roaming around on this site somwhere.
"If one advances confidently in the direction of
his dreams, and endeavors to live the life he
has imagined, he will meet with a success
unexpected in common hours."
- H.D. Thoreau