Vertex color is the color assigned to each vertex irrespective of the color of material. You can color vertices with [Vertex color] command. But you need to make vertex color available in material Panel. And it is not applicable to the object selected as patch.
When you click a vertex with pressing the shift key, its color changes to the currently selected color. When you do the same with pressing the Ctrl key, the color of the selected vertex is erased.
Vertex color is not supported in many file forms except MQO. Vertex color is mainly used for objects of low polygon in games.