Do I need a random geometry, material, or something else? - SceneKit, Swift

In my game, I add a new a tempLeftBox (an SCNNode with an SCNBox geometry, and an SCNMaterial) every 5 seconds or so along a straight path. The tempLeftBox has its own geometry and its own material. For each tempLeftBox that is added, the color of the box should be random. This random color should ONLY affect that ONE box that was just added. Not all of the boxes that are going to be added or have been added already. How should I go about doing this? Swift, SceneKit