pre_cleanup.py

import bpy
 
def gump(color_choice):    
    """
    go to vertex paint or edit mode and select a few faces
    then run this script
    """
    print('_'*80)    
    
    # find object
    my_object = bpy.context.active_object.data
    
    # find color map
    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)
    
    # make_paint_list:
    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 color_map, paint_list[:]
   
def color_vcm(color_map, paint_list):
    print("vertex color indices: ", paint_list)
    # paint vertex colormap indices using paint_list
    for i in paint_list:
        color_map.data[i].color = color_choice
    bpy.ops.object.mode_set(mode='VERTEX_PAINT')

ob = bpy.context.active_object

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

def face_masking():
    return ob.data.use_paint_mask

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