A little late for this post I am sure, but when doing a color match spray test it is best to use the substrate you will be painting (metal) with your Primer, Silver Base, top color and clear to get the best match.
Do not use cardboard, it is way more absorbent of most light frequencies than metal or plastic.
Anyone who doesn't believe this should try painting drywall with and without drywall specific primer/sealer. Paint absorbs into drywall differently than it does joint coumpounds.
Warning; the following story will take a few minutes but it is entertaining and will be worth it when you are finished.
I when I finished drywall in my construction/remodeling business up to about 2008 I would bid my drywall jobs with myself applying the Primer Sealer. I would get customers that wanted to cut corners that would want to do their own priming and paint.
I told them if they insisted and wanted me to do the job, they would have to sign a no fault agreement covering visible seams and texture/color differences between drywall board and seam coumpond. Otherwise they had to have a reputable painter prime and paint the work or they had no guarantee on the paint or surface match.
One couple signed the agreement, then when I was finished they painted the room with some cheap standard primer for repaints and Wal-Mart paint. Not a problem with the paint, but that is what they used.
The room looked so bad it almost made me vomit on my Wheaties when I went to look at the results following their work. The seams looked like stripes of a different shade than the bare drywall.
I asked about primer and at first they swore they used the primer I recommended. I said fine, let me see the can of primer you have left over.
At first they resisted, but after I picked up my notebook and both jobsite and digital cameras to leave they brought out the cans of primer and paint they used. When I asked who to them that primer would work, they said "the guy that mixed our paint at Wal-mart".
When I told them they violated our signed release they got nasty and refused to finish payment on other work I was done with a few days before that was bid on a separate contract.
I love arguing with idiots, but these people were beyond reasoning. I went to the courthouse and filed a Mechanics Lien on the home and property. When they received their copy, they called a lawyer and sued me for (yes they did!) Breach of Contract for the drywall work. I countersued for the lien, lost time and wages for preparing the case and court appearances.
In court their attorney argued that the paint technician at Wal-mart was an expert witness. Even the judge had to hide the chuckle he got from that one!
When asked for my qualifications as a contractor, my apprenticeship or journeyman status and years of experience, I simply said here are my qualifications. I do repair and remodeling for ******* who owns the local Classic Car and Corvette dealership, and said "You know how picky those guys are, and he has called me back for other jobs. Since he and most of the judges here are Masons I knew the judge knew him.
I produced all our signed documents, photos of the work before and after painting including one of the couple feeling the seams for smoothness, as well as of their paint materials and a transcript of the basics of our discussion of the problem.
When the judge admitted the paperwork as evidence, he nodded inconspicuously my way and asked if the plaintiff was finished. They answered yes.
I prepared to start my case, but the judge said he was already prepared to rule so I didn't argue.
He admonished their attorney for even bringing this frivolous case before him, and ruled in my favor as well as my lien and countersuit for lost time and wages.
They ended up paying me about $2500 more than if they would've left me alone.
Oh, and the judge called me back to chambers and invited me to join the Freemasons, which I respectfully declined. It still made a friend of him that day.
The moral of this story?
Substrate material does make a difference.
