3. In the Tone Curve panel, from the Color Control pop up menu select RGB, or each of the color channels individually.

You can adjust the luminosity of each of the colors or collectively. Individual colors are represented by a tone line in the graph of the same color. RGB collective adjustment is a white line.

If you need more room to make fine adjustments in the Tone Curve panel, you can resize the entire panel by click-dragging the lower right corner of the panel.