Pixelmator
Color Adjustments

Color Adjustment Layers

Open the Color Adjustment tool by clicking its icon in the list of tools or just type the letter (a) on your keyboard

Adjustment Layers are pretty simple you just click the icon that looks like a stack of pancakes at the top of the left side of the Color Adjustment tool and choose Color Adjustments as shown in the photo above. Once you have an Adjustment layer created use any of the three Selection tools to outline the area of your image you want to modify and then right-click the Adjustment layer you earlier created and in the box that pops up click on Add Mask. And now if you go back to Color Adjustments by pressing the letter A on your keyboard you can use the various sliders like Temperature, Tint, Exposure etc to edit the look of only that part of your image the selection applied to

And as you can see by the image above, you can make as many Color Adjustment layers as you like

You can watch a video on my YouTube channel that demonstrates how to use Adjustment Layers to alter the subject and/or the background of an image