System Information
Win 10

Blender Version
Broken: 2.79 daily builds

I think new vertex color alpha should be accessible from bmesh module, right? For now bmesh loop[color_layer] takes array of size 3, so no vertex alpha can be set (r,g,b,a).

Exact steps for others to reproduce the error
Run script. Line:
loop[color_layer] = bevelCol #only array of size 3 works
Wont take 4 params