Hey ridwan, I like the style of your painting. There are some things that I think would help you moving forward. The main issue I see has to do with the colors. They are coming off as dull because you are using black to darken and white to lighten. One of the things you can do to fix this is to color shift. Think about the temperature of the light because the shadow will be the opposite. In other words, warm light- cool shadows and vise versa. This will help you know how to color shift as you darken and lighten. I included a quick sketch to show you what I mean, I hope it helps :)