Gradients is when colors blur into each other. There's several of them in Photoshop already, but you can also download them online. I'll be using this pic:
1. Click on Gradient Tool, and click the colored bar on the top, and a little color map will appear. Choose the one you want. See here.
2. Make a new layer, Ctrl + Shift + A, and pull the pin over the pic, so that it's covered with color (see example). Now change layerstyle, opcacity and so on to what you think looks best.
I choose Hard Light at 100 %.
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