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.