I made an add-on that is designed to help make working with vertex color data a little bit easier. The main feature of the add-on is the isolate workflow, that allows you to isolate one particular channel of color data and work on it with all the normal tools. There are also tools to help you move data between channels and between different data types, so you can transfer color to weights, weights to color and even use UV data as well as copying and swapping individual channels of color data.
For good measure, I have also added a bunch of tools that help you work accurately with vertex colors, such as blur, remap, posterize, rgb to luminosity and more.
Note: This image shows the old 2.79 version of the addon, it’s quite a lot better in 2.80. I’ll try to update this soon.
2.80 quick start: Once installed, you can open the pie menu by pressing the ‘V’ key while in vertex paint mode. This exposes most of the functionality, but the right hand 3D View panel contains the full set of menus under the heading ‘Vertex Color Master’.
Documentation can be found on the Github page here.
Vertex alpha is supported, but only if you are using Blender 2.80, or one of the latest Blender 2.79 buildbot builds. Blender 2.80 (beta) supports vertex alpha, but currently does not render it in the viewport - however, you can use the isolate channel function of this addon to view or modify alpha, and it should be exported correctly when using export to FBX.
Feature suggestions or bugs reports are welcome, please let me know in the comments below, or on the issue tracker on Github.