This is a python script for blender that has tools to properly control vertex coloring, it support live updates from edit mode. Simply install into the scripts directory, then inside Blender split off a scripts window, and run from the Wizards menu. All the tools are self explanitory, if there is still confusion from the names simply play on a simple model like a sphere to learn the results. The operations are performed on any of the verts, edges, or faces you select, and it will work either at the vertex or face limits. Using GLSL you can see the results live and lit.
This little script has helped me quite a lot, thanks. I hope you update it for the latest blender too.