paint shade match problem-PPG

[Doc Dave]

About a year and a half ago I had my PPG store mix me up some urethane single stage, an antique color called Dodge deep puple blue, from the early 50's. It is for my Power Wagon frame off restoration I have been working on.
By doing some research I got the Ditzler code for it for the paint store to use, and all seemed good.
Well, now after I finished the sheet metal and primed everything, I have the sheet metal at the paint shop for top coating in their booth, and the painter calls me to say he finished the bed (looks great), but he needs two more quarts for the cab. I call up the PPG store, give them the code off the paint can they mixed, and voila, they send back paint that is obviously somewhat lighter than what the original was.
Everyone is scratching their heads. We can't understand why the "recipe" is not giving us the same color pretty close if not exact.
What I ended up doing after two attempts by them to get the right shade is to send the original correct first batch (still had a small amount of paint at the bottom) back to them, so they can use the Prophet system on it, and hopefully figure out what the deal is.
I would rather not repaint everything the lighter shade. It would be more expense for the painter's labor, plus is not very close to the stock dark blue.
Could the tints have changed in the can over 18 months? Could PPG have changed the tint characteristics?
Help!
Thanks

[Len]

I doubt that anything has changed, my guess would be that the paint was probably mixed improperly the first time. If they try to match the original paint you want to be sure to get TOO MUCH paint so that you have an exact match in case you EVER need it.

This is not an unusual event, a guy or gal that mixes paint all day will sometimes screw up when pouring and measuring the tints, this is one of the reasons why you should always order more than you need on the initial purchase.

[Doc Dave]

The painter called me today and said that the PPG store must have done some analysis on the original paint with their spectro machine, because they sent him some paint that looked the same as the correct first batch. SO, I am feeling much better.
Len's point is a good one. When I ordered the first gallon, being a novice I mistakenly thought that would be enough to do the finish coats. In the future I will always order more than what I think will be necessary, to avoid this problem.