As much as I love Coremelt (been their friend and beta tester for over a decade), you don’t need it at all for what you want to do. So here’s a mini tutorial (and I think I answered this on the Apple forum, also). This is the most used and basic way to do it. If there are chroma/luma/IQ issues, you may need to use a mask along with this to help out. In really bad cases, sometimes a third party tool can help a bit. But if this won’t get it, it’s going to take a lot of work. Here we go…
Pull up the Hue/Saturation Curves.
Use the first option, the Hue VS Hue.
Click the eye dropper.
Click and drag on the color you want to isolate.
Three dots appear in the Hue Vs Hue panel, the middle one is the color you selected, it has a vertical grey line through it.
Drag that middle dot up and down to change the color of your selection.
If your selected color is too close to other colors, drag the two outer dots closer to the middle dot to more closely isolate that color.
Remember you can drag the edge of the Inspector pane to the left to make it wider, which helps with this particular tool.