I have a 1992 Horizon Grey Metallic teg with USDM black moldings. I was planning on getting a set of JDM moldings, and then painting them with some color-matched paint. I don’t like how far my moldings stick out, and since I have to remove the old ones anyway, figured I would paint the new ones while they were off of the car. Should I practice on the old ones?
up to u man, is ur money, ur taste. 
You should always practice on old ones IMO. Before I cleared my tails I went to the junkyard and bought a broken tail light for 5 dollars to work on so I wouldn’t F up the real ones.
thats great thinking sparkman!
youve checked out how to do em right i take it. just remember a good paint-job is in the prep! and take your time! and have fun with it!