A detailed explanation of how Photoshop gradient maps are used

Photoshop's Gradient map command maps an equal grayscale range of images to a specified gradient fill color. For example, specify a two-color gradient fill, the shadow in the image is mapped to one of the endpoint colors of the gradient fill, the high light maps to the other endpoint color, and the midtones map to the gradient between the two endpoint colors.

Next we'll look at how to define a gradient map, and first open a graph.

The Gradient Mappings dialog box pops up when you perform the image > adjust > Gradient Map command or the layer > new adjustment layer > Gradient Map command.