Let us talk about that gas mask for a moment. There seems to be banding, and the shading is a squared feeling to it. Masks are round, not squared. The shading you made is as if you are shading a box. Not only is there banding but there are jaggy lines. The bottom curves of the mask especially. As for colouring, don't go full grey. Add in some blue or purple. The eyes.. Are you trying to make them round? Because if you were, you failed.
Johnny said to watch YouTube videos. I wouldn't do that personally.
I prefer reading threads in regard of shading and colouring.
Not only that, but I like to be inspired by arts from pixeljoint. You should check it out sometime.
Keep practicing.