Vertex color is the color assigned to each vertex irrespective of the color of the 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 a 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 supported only in a few file formats. Vertex color is used for objects of a low polygon in games, 3D scanned data and so on.
It changes a vertex color of clicked or dragged vertex.
It changes vertex colors of all vertices within a clicked or dragged face.
It changes the vertex color of each vertex within a clicked or dragged face.
It changes vertex colors of the vertices that pass nearby when dragging.
It calculates an ambient occlusion. After the calculation, a part surrounded other parts will be shown dark. The result is stored as a vertex color.
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| Before | After |
It converts vertex colors to a texture and saves it to a file.
You can specify output size, a filename, and selection parts; objects, faces or materials; to output.
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| Original | After saving a file |
When you open the above file, you will find the following texture image.
It converts an original texture to vertex colors.
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| Original | After conversion |