[Endoperez]

I quess it would be done by double-clicking on the color box, setting the color to white either by changing numbers or by putting the selector to the white corner.

To input one and only one white pixel, I think you have to choose a drawing tool (pen, brush, whatever) and choose a brush (in the advanced tools, for me undre the tool selection) that is Circle (1px) or Square (1px) or similar. Just make sure the painting tool you use is not set to partial transparency. See the slider in top of the advanced tool selection box.