Reversing light tones
The simplest tone reversal is an arch-shaped curve that reverses all the tones lighter than the mid-tone point of the image. Simply drag the top point on the line down to the bottom of the graph, then click on the mid point and drag it upwards. In the sample image used here, this reverses the tone of the sky, turning it from blue to a bright green/orange gradient. The darker tones of the foreground and the boat itself are more or less unaltered. By altering the height of the mid-point of the curve the overall brightness of the image can be adjusted.
While this technique is very simple to perform in GIMP, Photoshop and Paint Shop Pro, there is unfortunately no way to achieve the same effect in Photoshop Elements 8, because the end points of the tone curve are fixed. The screenshot below is from Paint Shop Pro Photo X2.