NEW IN R21
A volume can be seen as a clould made up of millions of tiny droplets. Each of the droplets is added in the direction of view and produces a corresponding density. The color gradient can be used to give the cloud a unique color. The areas with very little density can be colored using the colors at the left end of the gradient and more dense areas can be colored using the colors at the right end of the gradient. The distributed colors are also multiplied with the respective density and can depend heavily on the density within the volume with regard to their opaqueness. This is why the intensity of the assigned colors can be adjusted separately using the Color Strength setting.
Details about using the color gradient can be found under Color Gradient in Nodes.