If you have added vertex coloring to your plot visualization, you can add a colorbar which will show on a color scale the minimum to maximum values of data in the visualization.
To add a colorbar,
This colorbar can be resized and moved around the data space. You can also double-click on the colorbar to invoke the Visualization Browser which will allow you to change the values for the colorbar. For more information on the colorbar, see Colorbar, "Visualization Properties".