Use this function to tint the picture towards a pair of colors of your choice. The input picture is first converted internally to grayscale. All output colors are mapped to shades in that "hidden" grayscale step.
You can reach this function using Edit | Effects | Gradient Map in the Browser, or Effects | Gradient Map in the Editor.
The darkest shades in the picture will receive the color on the left of the gradient; the lightest shades get the color on the right. You can set the endpoints of the gradient by clicking the buttons at the two ends of it. You can add other colors of your choice in the middle of the gradient. To do so, either double-click the gradient or click Add. To change a color on the inside of the gradient, select the marker for it and then click Active. You can also move the marker to a different place inside the gradient. Clicking Erase will erase the selected color from the gradient; Erase All will erase all colors inside the gradient, leaving only those at the two ends.