You make the selective color red, then you erased everything but not her lips, right?

Yes. Selective Color lets you take a color, in my case the reds, and change just that color. So, when I made it "redder," only the reds were affected. The yellows, greens, blues, blacks, and other "non-red" colors were unaffected. I turned the "redder red" off everywhere but her lips.

And do you mean that you can add a "hide all" then paint with a red pencil? *iz confused*

Not exactly. The mask is just an "on/off" switch for applying the layer. In this case, the layer was me playing with reds but in the base you made up there, it was the sharpened layer you were turning off at certain points.

I did not paint on the mask in red. In fact, you can't paint on it in red. You can only paint on it in white (on) or black (off).

The "Reveal All" option adds a white mask, turning the entire layer "on." The "Hide All" option adds a black mask, turning the entire "off." After you've added a layer that way, it isn't unchangeable. You can still paint on it to alter portions as being "off" or "on."

Does that make sense?