I dunno for sure. I suspect that while the formula may be available for the correct color, the hue of the tints used may have changed slightly over the years which would result in what you are finding. It's also possible that the truck has been repainted since new to a slightly different color. The best bet would be to disregard the formula and custom match the existing paint on the door jamb.

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Name your dog Naked so you can walk Naked in the park.