The Random Color texture is a greyscale texture map that you can plug into a Tool Gradient Node to achieve random colors (Figure 1). The node creates random float values between 0 and 1 that are constant, but different for instances of the same mesh. This is particularly useful in modifying instance colors that may otherwise have the same material, such as simulating different shades of green of instanced trees in a common scene.
Figure 1: Using the Random Color node to control gradient colors across a series of object instances