Plot the colour value of any sample on RGB and Hue curves. While you make adjustments, you can plot colours as often as necessary to isolate the colours you want to modify.
To plot colour values when adjusting curves:
On the RGB curve, red, green, and blue vertical lines appear, representing the red, green, and blue colour channels.
On the Hue curves, a single vertical line appears, representing the colour on the hue spectrum.
The area of the curves contained by the selection box now occupies the entire Curve canvas.
The aspect ratio of the Curve canvas is maintained while zooming in and out.