The original background was a plain green wall… a plain background is really important for this next step.
Add a new layer, then go to the Gradient tool in Photoshop and pick a nice blend of colors, or design your own color stops. Generally I like the linear gradient for this effect (first one on the left). Pull your cursor at a diagonal on the blank layer… and voila… well actually not yet.. all you will see is the gradient.
Lower the opacity of that layer to around 50% for starters and go through your blend modes… usually Multiply, Overlay and Soft Light are the best options. Depending on the colors in your image… and the colors in the gradient.. you will get different blends.