material_from_vertex_color_map.py

import bpy
import random

# start in object mode
 
my_object = bpy.data.objects['Cube'].data
 
color_map_collection = my_object.vertex_colors
if len(color_map_collection) == 0:
    color_map_collection.new()
 
"""
let us assume for sake of brevity that there is now 
a vertex color map called  'Col'    
"""
 
color_map = color_map_collection['Col']

# or you could avoid using the vertex color map name
# color_map = color_map_collection.active  
 
i = 0
for poly in my_object.polygons:
    for idx in poly.loop_indices:
        rgb = [random.random() for i in range(3)]
        color_map.data[i].color = rgb
        i += 1

mat = bpy.data.materials.new('vertex_material')
mat.use_vertex_color_paint = True
mat.use_vertex_color_light = True  # material affected by lights

my_object.materials.append(mat)

# set viewport shading to texture to view this.

Trying to understand why you are not using the idx value on line 26, and instead are using i …