As the owner of a yellow/yellow z/28 that I restored, the seat covers in the pics appear to be repro covers from PUI.
They are bright yellow and are not even close to the original yellow that came from the factory.
I took a piece of the original material under the bottom of the door jamb windlace that was never exposed to light and sent it in to a company in Canada to have custom interior dye made. I then dyed the new seat covers and the new Al Knoch door panels to restore the original look. Very time consuming but it looks great.
Here is a pic of the back seat bottom with the correct color and the top back in the PUI yellow before dying.