Don't use a solid black outline, it just doesn't fit. Try to make the outline varied colors like the ref image you linked to.
I see banding on some of the edges, remove that.
Also, banding is when you got one color and a different color forming "bands" like this:
OOOOOOOOOOOOOOOOOOOOOOOOOOOOO
OOOOOOOOOOOOOOOOOOOOOOOOOOOOO