I really don't know how to do coloring. After spending lots of time and effort to finish the final version, I found this flat color version may looks better... I have to learn about the basic knowledge and technique of drawing all over again.
Anyway here is the PSD file. You can see how I struggled to make this piece look better from the layers.
https://drive.google.com/open?id=1iy0Ujx6pvEc32W085ev8PaxPwTGIovS3
Also, it would be very thankful if you could give me any advises.