Henry, metamerism is the condition not where it happens. It is what it is, and it doesn't matter if it happens where you will be driving it, where it's parked, or where it is painted. If the appears different under different lights, THAT is metamerism. You are right in that if the car is never going to see the conditions you have it under when you see this like your own shop lights, maybe it won't matter, this is true. The customer is never going to see the car in your lights and once outside it looks good and it looks good where the customer will see it, it may not "matter",but the metamerism still exists.
You and DBM are right, learning how to tint yourself is the next step. It is a steep curve but if you are going to be painting, you MUST know how to move a color around a little. Personally I have not had good luck with the spectrometer "computer thing-a-ma-jig" (more people know it by the thing-a-ma-jig than spectrometer you may be right
) I used one when I was a rep and we had one at work with the last paint supplier, it was "close" or MAYBE a "Blendable match" but it certainly didn't "match" the color in my opinion. Of course this could be operator error or simply not as good a thing-a-ma-jig as the guys you use have. I do know I had a Dupont "Chromavision" as I remember they call theirs.
gave a solid color for a metallic at one shop I serviced.
Learning to tint a color around is the best suggestion if you want to continue doing this stuff.