"The original interior was black."
Chuck,
That explains the black dash.
White has a tendency to yellow from sunlight.
But, now that you have mentioned that the covers are from Legendary, that explains a lot.
I sent the original cover to them when they got started with '65 Buick interiors. In return they sent me a set of covers for the use of the original as a pattern. The white that Legendary was using at the time was an off white. That is what Legendary sent back to me.
So, you are correct, it should be a bright white. That also is what the Buick dealer album has as a material sample.
Loren