I will be using this image….nothing special or remarkable about it, but it works for these purposes. Use this one or any image of your choice to try out this selective color technique.

There are two ways to begin once you have your image opened up in Photoshop. You can either apply the effect to the image or Layer >> New Adjustment Layer >> Selective Color. Personally, I like to duplicate my image (so I have an extra untainted layer should I still need/want it) and apply the effect right to it. So that is what I am going to do. Go to Image >> Adjust >> Selective Color. As you can see, there is menu of colors:

Let’s say we want to give the image some intense red tones. Select black from the drop down menu. Now (using the relative selection, and not absolute) I like to bring the black line up a few knowtched toward the positive side. You will notice that the areas of black get even darker and more pronounced. Nice, eh? For some red, move the cyan line toward the negative side:

To get some more red, use the neutral option. I positioned it like this:

Final image: