Are the L,a,b readings on an X-rite dependent on the mil thickness of powder?

A. Hello, I used an SP-68 for years and never came across the issue of the paint thickness (opaque to < 6 mils) changing the readings, but I used the Delta-E as my standard, not the individual L*A*B*'s. X-Rite does have a tech support site that you need to contact and get some of your questions about the machine answered, has it been re-certified at the proper intervals? Has the standard been re-certified?
It would certainly explain a lot of things if the paint has a batch to batch stability problem. I agree with the assessment that a good powder won't change color based on the thickness, but I have seen noticeable color differences(Delta E readings) between batch's at times. I can't say that I've ever dealt with a supplier that stated that the color would be met as long as I held a certain paint thickness, that's a little odd.

A. Colour control is not dependent directly on film thickness as it is on the hiding quality of the pigments used. I am quite familiar with the X-rite and colour control is one of my main areas of expertise, both paints and inks. Pigments such as black have very good hiding. Other pigments such as extenders and many organics have very poor hiding. Many are translucent. If problems are suspected with a batch the X-rite has a contrast ratio feature that in conjunction with black and white Leneta charts can be used to determine hiding. I'm not familiar enough with powder coating to help you develop a methodology.