Simple Addon for Blender, for setting your vertex colors in specific tones (with alpha)
Install like any addon:
Edit -> Preferences -> Addons -> Install
1. Travel to the Viewport
2. Select the vertices you want to change
3. Select the Tool, Toolbar
4. Select color
5. Press "Set Vertex Color"
6. Profit
Remember that you need to set up a shader beforehand to visualize the alpha channel in blender, it's commonly used the Color Attribute Node