Im not good with critique, but one major thing i personally avoid unless its intentional is using a pure black, it blends into the linart. Using a dark gray separates it from the outlines. As for the pants, the blue is really dark and without any highlights/shadows, it also blends into the outline too. It might be fine, but i would choose a slightly lighter blue.
As you develop you should also learn shading and highlighting techniques, as unless you are a master of color theory, lighting is one of the best ways to make something stand out.
I guess the last thing ill say is,most of the background is that dark red. Too me thats too much space not being used, i dont know what you could add, but thats just my opinion.