Gradient Overlays may be used with photo treatments, as well as on their own, to add or reduce contrast between graphic elements.

Figures 1 and 2 demonstrate the use of gradient overlays.
Figure 1 uses cyan (C:100, M:0, Y:0, K:0) to green (C:65, M:0, Y:100, K:0) Gradient Overlays.
Figure 2 uses magenta (C:0, M:90, Y:40, K:0) to Rotunda Orange (C:0, M:68, Y:100, K:0) Gradient Overlays.
How To Create this Effect

1) Create a shape container filled with a gradient of your choice. (E.g. cyan (C:100, M:0, Y:0, K:0) to green (C:65, M:0, Y:100, K:0) or magenta (C:0, M:90, Y:40, K:0) to Rotunda Orange (C:0, M:68, Y:100, K:0))
2) Place a black and white version of the image you want to be seen in the gradient overlay.
3) Set the blending mode of the photo on multiply for gradient to show through.