It looks great as is! I do have some tips though.
As for all shading, it's important to keep the light source and the forms of the figures in mind when you do shade. Some people have an easier time doing negative cel shading, where they cover the entire subject and shadow and "carve out" where the light is hitting. You may want to try that and see if it works better for you.
Consider the location when you shade and how the color of the shadows change based on where you are and what time of day it is. Purples, dark blues, and darker reds are good colors for basic shading, at least I think those colors are the most appealing.
You can also layer your shadows and add gradients to them for some dimension.
There is great manga artist Whyt Manga who has a multitude of videos on the subject of comic making. Here's a good starting place: