Curves

cmd ⌘ + K
Curves is one of the most powerful tools in Pixelmator.
With very few adjustments, you can adjust the over all contrast or tonal range, the local contrast or tonal range, and the color.

Choose Image ‣ Curves...

The horizontal axis in the Curves dialog box represents the current values of the pixels in an image or a selected part of an image. The vertical axis represents the tonal values after the curve is applied. The default curve is a straight line with an angle of 45 degrees. For each pixel, the input value is the same as the output value.
Click to reset values.