for the bottom tile I chose 4 shades of the green I wanted to use from light to dark.
along the bottom left and right edges of the tile, I drew lots of 2 pixel high vertical lines randomly choosing between the 4 colors, but ensuring that there were no 2 lines of the same color next to each other.
after doing this, I took the lightest color, and randomly, but evenly places single pixels of that light color over the tile, I then did the same with the darkest color, not really worrying about spacing it evenly between the light pixels, just evenly over the tile, then using the two mid-tones I filled in some ofthe pixels around the dark and light pixels to give the tile a little more depth, the darker midtone was placed around the dark pixels, and the lighter midtone around the lightest pixels.
if I was more serious about this, and not just making an example, that last step I would spend a decent amount of time on,making sure that the spread of light and dark was as close to even over the tile as possible. as it is in my example, there is a little too much dark.