there are several ways to do this but basically what you are looking for is a way to select a range or tone or colors and affect only them. this is also called secondary color correction. one of the more straightforward approaches would be to just use Hue/saturation, set it to Greens and not change the hue slider. if you need to expand the range you can do that with the sliders. this is also called secondary color correction. there are more precise ways to do that. for example the lumetri effects comes with a secondary color correction module where you can set the mask with more precise controls.
The effect demonstrated in the Film Riot tutorial was achieved and demonstrated using some products from Red Giant. To pull the same thing off using the effects that come standard with After Effects requires some understanding of the color grading process and the use of Levels, Curves, Hue and Saturation, Channel effects and a bunch of other effects under the color correction sub menu in effects. The exact technique depends on the footage, as was explained in the tutorial you referred to. You may also need to separate your actor from the scene using some kind or masking, roto, procedural matte or keying technique. The most effective way to do that also depends entirely on the footage. Roei started you down one path, there are many you may choose. If you are really new to AE then you should start here: Basic AE. If you have some experience but are lost on the use of some of the effects try typing the name of that effect in the Search Help field at the top right corner of AE and study up. There are literally tens of thousands of possible switches to be thrown in AE and nobody knows how to use them all without years of study and practice. You have to read the manual and practice a bit.
Did you ever solve this issue?