Contrast! you have 4 shades of green (if I counted right) and they ALL are very close. Give way more contrast to your palette. And don't hesitate to shift the colour hue across the palette. For example, go for a more "blue" green for dark places, and a yellowish one for light. As for the outlining, I don't use it myself so I'm not the best to give advices.
Last step of the animation (stepping forward, left foot) is a bit hard. An additional frame would be nice.