Previously the colors were just uploaded as white, the default value.
Even if they aren't interpolated properly, it is still helpful to see
the colors. At worst, the unaffected parts of the mesh will still look
right.
A previous commit made vertex colors interpolate properly, but
face corner colors will still reset to their default value.
As a reminder, only color and byte color attributes are currently
supported for the specialized PBVH drawing.
Pull Request: https://projects.blender.org/blender/blender/pulls/113333
73e345f85f