There are several different ways to use gradients in Adobe Photoshop. There is Gradient Fill and the Gradient Tool, which sort of add a splash of color, tones or light and dark to your images. Then there is the Gradient Map.
All three work differently. The Gradient Map actually maps out the different tonal ranges within your image and applies the Gradient Map to that. If you choose, say, cyanotype or sepia gradient, the darkest area of your image will take on not only the tonal range, but also the color of that gradient.
Where to find Gradient Maps?
In the Adjustment Layer panel (the circle with a dark half) you then select the Gradient Map (toward the bottom of the list). This will set up a Gradient Map on a new layer on top of the current layer you are working on. This is a great idea as it allows you to blend the Map in to your image further as you wish.
How to use a Gradient Map?
Once you have your new Gradient Map layer, it may be just black and white for now, click on the slider the bring up the presets. There should be several rows of different options, which you can click. You can import your own or create your own by click on the little toggles on the slider.
A color picker box should pop up. You can change every color in the slider and then saved it to your swatches.
Dial it in for the look you want
Now that you have your Gradient Map and the colors you like, you can alter the blend mode and the opacity. On the below image I have used a sepia map, soft light and 50% opacity.
When looking for a subtle tonal change, or something a little more dramatic. Or perhaps, you want to boost your contrast a Gradient Map can offer a terrific range in colors and tones, as opposed to just putting a colored gradient over the top of your image.
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