Set the color to white, but not drawing white.

[Ofnuts]

The purple corners in the swashes indicate that the colors are not available for your image. It is likely that your image is color-indexed (check the title bar) because it is a GIF. In that case Gimp coerces the color you want to the closest color alreadyin the colormap. You can move to full RGB (Image>Mode>RGB) to alleviate this, but the image will be color-indexed again when exporting to GIF, and if the color map was already full on on the initial image, the re-indexing may cause color shifts.