Look at any of the many guides floating around on pixel art color theory. I specifically recommend reading it from the context pixel art, because a lot of guides use limited resources (3-4 colors per object), which then places emphasis on things like contrast, effective colors, and composition.
Your grasses just look like blurry brushstrokes, because that's exactly what you've done - you need to refine it and add detail if you want to make it pop, and this is true of any artwork, not just digital drawing.
