If you go to Tools > Adjust Color in Preview, flip the black and white sliders around and then play with the slider in the middle until you get the correct colour. It's easiest to do it with your screen inverted, then once you get back to the original colours in the image you know you've inverted it properly.
Sorry, they're unlabelled. They the left and right sliders located just under the picture color profile; above the "auto levels" button. You just pull leftmost slider to right, and rightmost all the way to left.