I'm sorry to say I find this tutorial very weird. As others have pointed out, the inverted layer is an unnecessary step, it has no impact on the final result whatsoever. It only confuses people new to Photoshop. Basically this can be done with hue/saturation set to color blend mode and levels with some masking. My honest opinion is that this should have been free from the beginning as it's only basic Photoshop color correction.