another exercise that'd be worthwhile would be rendering the mushroom at different sizes, sometimes you'll discover too large and there are too many pixels to clearly define something without more colours, too small and there just isn't enough pixels to define the details, and sometimes you will find a perfect resolution where you can imply details with enough colour and shape information to imply texture
something like this would be on the smallest end of the scale of practical sizes
to further elaborate why I am making this point, if you can do non-pixel art at the same resolution that reads well, then you should question how much purpose there is in doing it in pixels
quick paint over, and the bottom left is rapidly reduced to 13 colours, just to point out, if you plan on having alot of detail, then higher resolutions are of course viable, but keep in mind to consider the resolution, it's always detail Vs amount of pixels/size. one has to justify the other. I think you could depict the same amount of detail in much less pixels, maybe try one at 50%