In case anyone has recently tried this guide…
… and didn’t manage to get the vertex channel transfer to work in the new Blender Color Attribute UI / workflow, here’s the way to go in newer Blender versions:
The Data Transfer modifier transfers to the default vertex color channel named ‘Col’. If you want to transfer a mask vertex channel to a mesh copy containing the vertex colors in the default ‘Col’ channel, then do this:
After adding a new Color Attribute channel (type: Face Corner ➔ Byte Color) you need to rename the already present ‘Col’ channel to a different name (e.g. ‘Colors’), then rename the newly added channel to ‘Col’, so the Data Transfer modifier will overwrite the new ‘Col’ channel with the mask from the other mesh, in stead of overwriting the default ‘Col’ vertex channel containing the colors.