Quite nice what you got going on so far.
You do not seem to optimise your colours at all. Having full control over every single colour is one of the biggest benefits in pixelart.
Yours: 69 colours - My edit: 32 colours
This of course is very extreme and perhaps a bit more monochromatic in places, but it is just to demonstrate what you can do. As a middleground you could so something like 40ish colours and it would be closer to the original colourwise. Back in the days stuff like this served a purpose because computers could not handle too many colours at once. Nowadays you still need it something when you work on stuff like the Nintendo DS. But for many of us it just became good habit and I for one think it's fun.