It's 5V confirmed and supplied from both sides. Still, "natural white" is more like "very warm white" whatever White LED algorithm I select. I suspected that I received RGBWW instead of RGBNW, but tried a second stripe purchased months later from a different supplier, same result. For the moment I am using just White off White off, not really why I changed my setup to RGBW...
Anyway, I noticed that RgbToRgbw.cpp applies quite significant adjustments to the white channel for cold and warm white stripes. Given that, I would expect that even natural white requires adjustments with Fx!=1. Just wondering...