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Colormaps are used to show details in monochrome (greyscale) images, by showing pixel intensity values using a wide range of colors instead of just from black => white. This makes finer details easier to see, although it may make it harder for colorblind viewers to read depending on the colors chosen. See https://publiclab.org/ndvi-gradients for a collection of common and useful colormaps by @cfastie. Some colormaps are not linear, meaning they use a set of color changes in one region of the scale to emphasize features with approximately that brightness.
Colormaps are also known as "lookup tables" or LUTs, and the process overall is sometimes known as "false color" (alongside other color compositing techniques).
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