White evens out the wood grain & color variations to provide an easier and more consistent solid color with less color coats. Basically, not matter what wood they used the final color comes out the same. It also brightens light colors on dark woods.
Fender custom shop still does white under all standard solid colors, silver & gold under candies and pearls, black under color shifting colors. All done with nitro.I don't know if Fender does it, but with lacquers, it's common to shoot a silver or gold undercoat over the sealer to reflect light back up through the surface. I expect white would do much of the same with translucent lacquers or poly as the top coat(s).
There's your problem. Your wood grain is telegraphing through the paint as "shading".
Does anyone out there know when Fender started using a white primer under the color coat?