Click the control or press C to activate.
The Cyan / Red control, part of the Edit Color Effects control group, allows you to change the tint of the effects you are editing to be more or less red in color. The changes will be shown in real time in the Preview Viewport.
Move the control to the right (toward the red icon) to make the effects more reddish-colored, or to the left (toward the turquoise icon) to reduce red.
Note that this control affects all of the effects under edit simultaneously - Primary Spikes, Secondary Spikes, Soft Flare, and Ring Flare.
This control does not appear when editing a grayscale image.