vertex_color_map_setting.py

import bpy

# instructions:
# "Use face masking mode, while in Vertex Paint mode"

def make_paint_list(my_object, faces):
    # paint_list will contain all vertex color map indices to 
    # be used for overpainting.
    paint_list = []
    i = 0
    for poly in my_object.polygons:
        face_is_selected = poly.index in faces
        for idx in poly.loop_indices:
            if face_is_selected:
                paint_list.append(i)
            i += 1
    return paint_list

def do_painting(color_map, paint_list):
    print("vertex color indices: ", paint_list)
    for i in paint_list:
        color_map.data[i].color = color_choice
    bpy.ops.object.mode_set(mode='VERTEX_PAINT')
 
def find_and_paint(color_choice):    
    my_object = bpy.context.active_object.data
    
    # use active color map, create one if none available
    if not my_object.vertex_colors:
        my_object.vertex_colors.new()
    color_map = my_object.vertex_colors.active
    
    # find selected faces
    bpy.ops.object.mode_set(mode='OBJECT')
    faces = [f.index for f in my_object.polygons if f.select]
    print("selected faces: ", faces)

    paint_list = make_paint_list(my_object, faces)
    do_painting(color_map, paint_list)

def vertex_paint_mode():
    return (bpy.context.active_object.mode == 'VERTEX_PAINT')

def face_masking():
    return bpy.context.active_object.data.use_paint_mask

if vertex_paint_mode() and face_masking():
    color_choice = (0.2, 0.3, 0.7)
    find_and_paint(color_choice)
else:
    print("Use face masking mode, while in Vertex Paint mode")