If users set "white"color (index color: 7) to layers, then the "white" can be shown as "black" color automatically if the background is white in order to show the drawing easier. On the other hand, if the background is set to black, and then black color will be shown as white color.
However, the color(255,255,255) is the real white color, it doesn't matter how background color changes, it is always shown as white. So if the drawing with real white color was printed on white paper, it will not shown.
The solution is to use the index color instead of the true color.