The tone curves can be used to adjust the overall contrast of your image. You can also use this tool to adjust RGB by selecting each colour at the top of the graph.
In this example, I’ll use the tone curve for contrast purposes. Clicking on the line in the centre of the graph, you can maneuver each point and create new ones. To lighten, bring a point towards the top. To darken, bring a point towards the bottom.
Nice clean contrast has a point at both the top and bottom to create a gentle S-curve. You can see that mine is incredibly subtle in this image.