I'm not sure where or how people saw the promo material, but on my phone and on my PC it always looked sky blue + light neon green/yellow. To me, my P6 looks exacty like the box it came in, which looks just like what I saw online.
As OP mentioned, Pixel 6 Seafoam back-glass has a tendency to look white, but it very much tends to depend on the color temperature of the lighting. Maybe it's why Google calls it only "Sorta Seafoam"? Under warm lighting mine does match the box & most Google online renderings/photos. Probably won't matter if I put an opaque case on it anyway!