Here are writing samples of the Pilot Choose on several different surfaces: paper that has been coloured with craft acrylic paint, watercolour pencil, a Sharpie marker, and pencil crayon, as well as paper cut from a magazine and card stock.
When writing on all of these surfaces, the white ink seems to be absorbed into the paper, resulting in an ink colour that appears pale grey rather than white. The only way I can get the line to actually look white is to repeatedly trace over the same line. The effect is least noticeable on the smooth, non-porous magazine paper, but it is slow to dry on this paper and so is prone to being smudged. I also found it hard to obtain a smooth, even line on the magazine paper.
The surface that the Pilot Choose seemed to write best on was the craft acrylic paint, but even on this surface the colour of the ink appears light grey rather than white.