Some of you guys seem to have anger management problems. If you respond to everyone that disagrees with you by offering physical violence, you'll have a rocky path through life.
You can't expect everyone to agree with your taste. Some people will inevitably think that what you want to do is silly, ugly, whatever.
What you SHOULD be able to expect is that anyone that you are paying to do a job for you will NOT give voice to these opinions in front of their patron or his representative, your friend. The guy at the shop is clearly a jerk. Jerks are all over the place and part of the reason they are jerks is that they often don't know when to shut up.
Let them finish the paint job and have your buddy make sure it is up to your standards, but don't use them again, and once you have the car back, tell the owner that you will not be using them in future, nor will you be recommending them to any friends, and then tell them calmly why not. I expect that any business owner will have words with their tactless employee pretty quickly.