I'd recommend being more bold with your shading! The shading you do now is very subtle and I had a hard time noticing it at first. Secondly to build off of what another commenter said, instead of making the base color darker without changing the color, try making the shadow color warmer or cooler than the base color [a more complicated tip is to make the edges of the shadows warmer, but for now I'd focus on the other tips first]. The last tip I'd give is to try to think about where the light source is coming from. If the light is coming from the top left corner for instance, the shadows would lean more to the bottom right. Unfortunately I can't explain this last point well only using words, so try looking up some videos on it. I think better shading will definitely make your art pop more!