This is just a vague idea, but I think is has potential.  If you have seen any of my other projects you will know I am a real tight wad, so this is defiantly thought of with a tight budget in mind.

The bulk of the device is a delta 3D printer that has a bed size of at least 5x5 inches.  This would have a 3D printed bed on the top of the bed that would have a hand shaped detent to rest your hand in for rough positioning, and a hole to hold a bottle of fingernail polish.  The entire hot end would be removed and replaced with both a camera and a fine paint brush.

The idea is the camera would use AI to pick out fingernails from fingers and guide the brush.  It should also be possible to detect and compensate for slight hand movements.

I think a raspberry pi may have enough power to run the show.

Thoughts and ideas welcome...