So first of all I am not an expert, so that is far from an expert opinion I just went ahead and did some minor adjustment that in my opinion give it a little bit more depth and atmosphere. I also tried to keep the changes simple, so that it wouldn't require a long time to get better in terms of pure skill, but more in a way of understanding how to work with what you already can do.
So as I said the changes are not big and I did not alter the style of shading or design, just the overall look of the painting (kind of sloppily too sorry was just a quick attempt). Basically I only altered your values, that might be a little bit more clear when we take a look at a greyscale version.
When we take a look at your painting you can see that there is very little contrast and our eye is drawn to contrast (best way to check is always a quick black layer on top of the whole piece set to color so you can check how it looks in black and white). It is hard to see form or depth when there is too little contrast and it appears flat.
The first thing you might want to work on is to get more bold with your values. try to let the character stand out from the background, you can also add a rimlight to make the cut more clear and especially in that piece with a bright light source directly behind the character that is a really nice way to make the character stand out more. You could push the contrast even further than I did there and add some bright highlights, especially on the helmet where the viewer is supposed to look i presume. I lightened up the background a bit and really darkened the character and especially the foreground elements. The closer an element is to the camera the darker it will appear to be.
As for the color version don't be afraid to really use the color of your light source. The orange lights on the helmet will make the whole material appear in a more orange color than just grey.
I hope that was somewhat helpful to you.
