I will be using this image for the tutorial:


Go to Image >> Adjustments >> Threshold and play around with the settings until you achieve a nice result.

Your image should now look something like this:


Now, in the Layers window (if a box that says Layers, Channels, Paths is not present, go to Windows >> Layers and open it up) right click on the layer of your retro-fied image. Go to Blending Options >> Blend Mode >> Overlay.

Now create a layer underneath it ( by going to New >> Layer >> New ), and using your paint bucket tool or gradient tool fill that layer.

I used the gradient tool, and this is my end result: