To get a good color match with any brand you need to compare to the variants
for that paint code. Some codes have several variants and can vary a lot.
My jobber carries 3 different brands, one of them is Omni, I select
the brand by which one has the closest variant, sometimes Omni is the one,
sometimes one of the others. They are all different brands and they all do just fine.

The last time I used Omni base it was terrible. Kid down my street had primed his truck with a dark gray and had me pick up Omni (he was on a non existent budget) in a white. I had to plow on 6 coats to get coverage..... 6!!!!! I barely had enough.... If that was Deltron I'd have been done in 2......

When I repaint my C5 I plan to have all the panels off the car on stands in the paint booth to spray all at once side by side while using Spray out cards then reassemble the car.
Their are parts on the car that cant be removed or installed easily like the Rocker panels and the roof pillar so they will be painted separately before assembly. I plan to use the Spray out cards for these pieces to get a good match.

The way I see it the OP problem is parts were sprayed off the car and now don't match each other while spray out cards probably would have helped prevent the problem neither panel probably matches the rest of the car.
just my opinion but they should have been painted on the car so to match the cars paint as close as possible while at same time performing Blends to the adjacent panels to get it all to match more closely over all.

No? opinions please?