如何制作代表我z值的色谱图例？

I have managed to use the grey scale command to make a 3D scatter plot with the third dimension representing a color gradient. I have hit a wall when trying to produce the legend that coincides with it though. I already have the range of colors I want I just dont know how to tell R to make them into a legend. Here is my code. I have managed to use the grey scale command to