setting_vertex_colours_once.py

import bpy

obj = bpy.context.active_object
mesh = obj.data

if not mesh.vertex_colors:
    mesh.vertex_colors.new()  # creates the map called 'Col'

# color_layer = mesh.vertex_colors.active
color_layer = mesh.vertex_colors['Col']  # must exist first

face_colours = [
    (0, 0.5, 0.5),
    (0.5, 0.5, 0),
    (0.5, 0, 0.5),
    (.4, 0.4, 0.2),
    (0.8, 0.1, 0.8),
    (0.2, 0.2, 0.1)
]

i = 0
for poly in mesh.polygons:
    for idx in poly.loop_indices:
        color_layer.data[i].color = face_colours[poly.index]
        i += 1

I have more examples here: http://blenderscripting.blogspot.nl/search/label/Vertex%20Color