When you use scatter plot, you set a color for both face and edge. In the official documentation you can find an additional parameter, edgecolors, which allows setting the edge color.
edgecolors : color or sequence of color, optional, default: None
If None, defaults to ‘face’
If ‘face’, the edge color will always be the same as the face color.
If it is ‘none’, the patch boundary will not be drawn.
For non-filled markers, the edgecolors kwarg is ignored and forced to
‘face’ internally.
So, after all, you need only plt.scatter(x, y, c=colors[i],edgecolors="black")