Here is today's coat of yellow, yesterday I painted the black rings. Tomorrow I'll do a run with red and black on the tip. If you screw something up you can always remove the paint easily with a cloth and Acetone. I'm not very good at hand painting things like this where you basically need a magnifying glass. I'm used to 'can't see it from my house' level of quality. I did some research on this part before I jumped into paint it. There are a couple of different tips of the arm from different movies. I'm going with this photo below.
So I'm on my way to make my CPU arm like this one during the end of A New Hope. But my tip is different but maybe in the future I can machine that type of tip and upgrade. You can always upgrade as you never finish a droid. You just keep upgrading them with better parts!