Curves
Curves are a way to adjust colors on an image by using a curved line instead of a slider.
Nodes are the points on the curve. Moving the nodes up and down changes the brightness, and moving the nodes left and right changes the contrast.
A negative slope inverts the colors. Nodes can be added and removed, for fine tuning of midtones.
The individual RGB color channels can have their own curves. These adjust the color balance rather than the brightness and contrast. A negative slope will only invert that color channel. Red becomes cyan, green becomes magenta, and blue becomes yellow.
Adjusting the curves on each separate channel can have some interesting effects. Playing with curves and experimenting can be a source of interesting color palettes and inspiration.