You can find them in two locations, image menu and also layer menu and adjustment layers. They can be used to radically change the color or add subtle changes.
They can be used to create amazing colorful designs using GRD presets as a source. The destructive effect can be found in the image menu and adjustments, the non destructive in the equivalent in the layer menu and adjustment layers. Select a preset from the preset picker and apply by clicking OK. For the non destructive you have access to a panel that can be accessed via the layers panel (double click the entry) and the preset can be changed at any time along with the blending and opacity via the layers panel.
How do they work ? They work on a black and white gray scale gradient. Go to the map properties and the left side is the black side and maps to the black or darker parts of an image. The right side is the white side and maps to the lighter and white parts of an image. So change the preset and the image will change accordingly as it tone maps your preset to the image.
It uses the source presets and these can be any of the presets supplied with the application as well as presets created by yourself or third party ones. With the most recent releases, you now have 100s of presets to choose from to modify your image
The tool maps the tones of the image to the presets. The darkest parts of the image will be mapped to the far end of the stops; the lightest parts of the image will be mapped to the other end of the preset stops. Of course, the preset can have more than just two stops and you can use this to create truly surreal effects with 10 or 15 stops. The more and more stops you have though th end result can look pretty weird.
Apply it via the image menu and then go to the edit menu and fade command to fade the it. You can then use different blending modes such as darken or difference as well as change the opacity of the effect. The result can be quite unusual. This is only available with the image menu option. You can also use the history brush to apply a faded effect to a localised area
Anyway, the presets / maps often results in some very interesting color effects that can be further enhanced by additional modifications / adjustments such as changing the end result to sepia via HDR toning etc. You can create far more interesting effects by using multiple layers and other effects and then combining those. You don't have to use the same preset either. You can create some interesting and surreal effects just by adding the map twice using the same preset but you don't have to use the same one and you can also intermix it with other filters and adjustments as well as using the blending modes as mentioned.
You can also add effects to the image before you add any of the color effects via the adjustments. Effects such as blurs and shape blurs work very well with the adjustments but many others are just as effective
Different applied presets will result in a totally different image so any of the default presets can result in some very interesting colors. Or just use the same preset and use the reverse feature but all that does is swap the stops in the preset. You can achieve even more extreme imagery. You can add loads more stops. Go to the adjustment (image menu or layer menu) and click on the editor. Add more stops by clicking on the preset bar to add colors such as 10 or more stops. Click OK. The results can be quite metallic like or perhaps posterization in many cases but that depends on the image and if the image is slightly blurred then the results will create unusual surreal glows. The image can then be further modified by other adjustments such as HDR toning.
The images can also be further manipulated by tools such as the oil paint filter and camera raw filter as well as combining with two applications of the adjustment along with HDR toning. Go to the image menu and apply the adjustment and follow that with the HDR toning (found in the same menu). Go to the filter menu and oil paint filter and camera raw filter to tweak the color in all kinds of ways.
The feature is useful for creating interesting text effects such as distress and inner glow and sharpening effects. The start point (for me) is often to apply some text and and blur the text (a little or a lot) and then apply the adjustment. It does not use the transparency information, it is purely a color feature. It can be made up two or more of color stops to create stunning color effects as well as distress effects. You can create all kinds of contour effects by using multiple stops such as 10 or 15 in the preset applied.
You can also use gradient map adjustment layers with masks to apply the effect to one part of an image Go to the layer panel and apply that new adjustment layer. Select the mask thumbnail on the right of the map layer entry. Add or remove from the mask by using white / gray / black as well as using effects such as blurs or twirls etc as well as changing the brightness levels by using image menu levels, invert, etc. You can even combine it with layer effects to create weird bevels and shadow effects
If you are using the image menu option, any selections added will limit the range of the effect. Anything outside of the selection will be unchanged. If you are using the layer option, the same will happen but the selection just creates a mask that can be further manipulated as described above.