Paint on copper pipe leaching into my water supply?
The Water is inside the pipe. The fitting you have on the left side connected to the painted pipe should be making a tight leak proof seal there should not be any paint at the point of inner seal. If you are concerned - remove the connection and sand the paint clean it and remove it then reconnect your fitting. 

If the paint is lead based - you can try a lead test. Be forewarned that what is coming from your utility will probably be more than what you have from that paint area if anything at all.

Paint is not leaching from the outside of the copper to the inside of the pipe either. 

Just thought I would add this picture to help you - see the insert sleeve that fits on the inside of the pipe - the white nylon piece. This seals the inside from the outside.