The purple corner is not really a second color, it is only telling you that the color is not possible in the image. This usually happens because either your image is grayscale (so you color will be converted to gray) or your image is color-indexed (and your color will be coerced into the closest color in the colormap).
You can void this by making your image full RGB (Image>Mode>RGB) if this makes sense. Otherwise, pick a color which is compatible with the image.
You can void this by making your image full RGB (Image>Mode>RGB) if this makes sense. Otherwise, pick a color which is compatible with the image.