you have many browns and tans that are very similar. try to avoid this in pixel art. either up the contrast between similar colors, or do what is called hue shifting:
this example isn't done pretty, but it shows the idea. instead of just having all greens of similar hue but just darker and lighter greens, shift the different greens to new hues. a common approach is as a color gets lighter, shift it to a warmer, yellower hue, and as a color gets darker, shift it to a more purplish/blue.
in pixel art you usually want all your colors to stand out from each other