I was thinking more of regular geometric shapes. For analysis I would take a look at PS that has a really good one and probably the best choice as it goes all around the pointer and displays perfect over too.
I would do something more focused over the mouse pointer and not inside the icon or under your hand.
a simple idea over the mouse.
as for the black border, maybe 50% grey instead though I am not sure. Any color you choose it has issues even if you do a flip colors blendy method for the colors underneath. 50 is stable and half way. donno. Regardless there cant be black between the current and the previous color only around I think.
Color inside the picker icon is not good though and the widget cant be too close or it will overlap the mouse pointer with lag on certain computers as it follows. but a good thing is that you can make regular shapes like squares, ellipses , polygons and beziers I think. it would not be out the table of considerations but then everything has to be inside a square after.