The idea behind Color Theory is to use complementary Colors because they work so well with each other visually. You will find them on opposite sides of the Color Wheel from each other. If you want to use more than three Colors, it is called a Color triad.
Color.Adobe.com is the resource we use, but there are plenty of other places to find great Color Wheels as well.
Applying Color Theory to Images
We used the screenshot from Adobe and added it to the actual image. This allows us to sample colors straight form the Color Wheel!
Create a New Layer and go to select -> Color Range. We start with clicking on the magenta Color in the image. Then, use the Eyedropper or Brush Tool to sample the red from the screenshot and fill in the selection. Change the blending mode from normal to color so that the shading comes through.
We do the same for the blue in the girl’s jean jacket, and add yellow to the Background. This gives us the complete triad of colors with the red, blue, and yellow.
Don’t be afraid to lower the Opacity of the Colors, because they will likely come out extremely bright and over the top.