Made a search in the ref manual for " negative " found this on page 594 , never tried it, will make a test
Invert
Use the
Invert function to use the color or lightness which is complementary to the one you originally
specified.
Channel - Use the Channel list box to select a color, luminance or alpha channel to which you
want to apply a function. The following channels are available:
None, RGB, Red, Green, Blue,
Luminance, Alpha
.
Blend with Original - Use this transparency slider to create a transition between the inverse of an
original image and the original image itself. Move the slider to 0 for a opaque inverted image;
move it to 100 for a opaque original image.