Vertex color from first material slot with baked cavity pass.

gistfile1.txt

import bpy
import random
#matcol =  bpy.data.materials["Material.001"].diffuse_color
#for m in bpy.context.object.material_slots:
     
for obj in bpy.context.selected_objects:
    bpy.ops.object.mode_set(mode='OBJECT')
    matname = bpy.context.selected_objects[0].material_slots[0].name
    #specialColor = bpy.data.materials[matname].diffuse_color
    specialColor = bpy.data.materials[matname].node_tree.nodes['Principled BSDF'].inputs[0].default_value
    # start in object mode
    mesh = obj.data
    if not mesh.vertex_colors:
        mesh.vertex_colors.new()
    color_layer = mesh.vertex_colors["Col"]
    # or you could avoid using the color_layer name
    # color_layer = mesh.vertex_colors.active  
    i = 0
    for poly in mesh.polygons:
        for idx in poly.loop_indices:
            #r, g, b = [random.random() for i in range(3)]
            #color_layer.data[i].color = (r, g, b, 1.0)
            color_layer.data[i].color = specialColor
            bpy.ops.paint.vertex_color_dirt(blur_strength=1, clean_angle=1.74533, dirt_angle=0, dirt_only=True, normalize=False)
            i += 1

    # set to vertex paint mode to see the result
    bpy.ops.object.mode_set(mode='VERTEX_PAINT')