First, open up any image you want for Photoshop. I've found this works better when your images are grey, so if they're not, go to Image >> Adjustments >> Desaturate. I will be using this photo of Beyonce:

Create a new layer in Photoshop. Go to Layer >> New >> Layer. Choose your Gradient Tool. For this tutorial, I will be using a blue/purple/magenta gradient. Just drag the gradient across your image.

Now, on that layer, fiddle with your layer modes. It should be on 'Normal.' Change the Blend mode to whatever looks best. I find that 'COLOR' works nicely.

This is what my image looks like now (setting at Color).

This is what my image looks like when I changed it to a rainbow gradient, set at Color.