Set specific vertex alpha value

Support Modeling
1

Hi,
How do I set a specific alpha value to the selected vertices?

2 
from bpy import *

mesh = context.active_object.data
vertexColour = mesh.vertex_colors[0].data

for i in range(len(vertexColour)):
    v = vertexColour[i]
    v.color = [1.0,1.0,1.0,0.0]