A draft of an simple add-on that colorize faces of a mesh based on this tutorial https://www.youtube.com/watch?v=WnZX--idSQI&lc=UgzNlDw63xzAORn-3Q14AaABAg

colorize_faces_op.py

# give Python access to Blender's functionality
import bpy

# give Python access to Blender's mesh editing functionality
import bmesh

# extend Python functionality to generate random numbers
import random

# import the Operator class from the bpy.types module
from bpy.types import Operator

def get_random_color():
    """generate a random color"""
    red = random.random()  # creates a value from 0.0 to 1.0
    green = random.random()
    blue = random.random()
    alpha = 1.0
    color = (red, green, blue, alpha)
    return color


def generate_random_color_materials(obj, count):
    """create and assign materials to the object"""
    for i in range(count):
        mat = bpy.data.materials.new(name=f"material_{i}")
        mat.diffuse_color = get_random_color()
        obj.data.materials.append(mat)


def add_ico_sphere():
    """add an ico sphere"""
    bpy.ops.mesh.primitive_ico_sphere_add(subdivisions=3)
    return bpy.context.active_object


def assign_materials_to_faces(obj):
    """iterates over all the faces of the object and
    assigns a random material to the face"""

    # turn ON Edit Mode
    bpy.ops.object.editmode_toggle()

    # deselect all faces
    bpy.ops.mesh.select_all()

    # get geometry data from mesh object
    bmesh_obj = bmesh.from_edit_mesh(obj.data)

    # get the number of materials assigned to the object
    material_count = len(obj.data.materials)

    # iterate through each face of the mesh
    for face in bmesh_obj.faces:

        # randomly select a material
        obj.active_material_index = random.randint(0, material_count)

        # select the face and assign the active material
        face.select = True
        bpy.ops.object.material_slot_assign()
        face.select = False

    # turn OFF Edit Mode
    bpy.ops.object.editmode_toggle()


def apply_materials_to_faces(obj, material_count):
    """iterates over all the faces of the object and
    assigns a random material to"""

    generate_random_color_materials(obj, material_count)

    assign_materials_to_faces(obj)


class MESH_OT_colorize_faces(Operator):
    """Create a new Mesh Object"""

    bl_idname = "mesh.colorize_faces"
    bl_label = "Add Colorized Mesh Faces"
    bl_options = {"REGISTER", "UNDO"}

    @classmethod
    def poll(cls, context):
        return context.active_object is not None and context.active_object.type == 'MESH'

    def execute(self, context):
        if not context.active_object:
            return {"CANCELLED"}

        apply_materials_to_faces(context.active_object, material_count=10)
        return {"FINISHED"}


class VIEW3D_PT_my_custom_panel(bpy.types.Panel):  # class naming convention ‘CATEGORY_PT_name’

    # where to add the panel in the UI
    bl_space_type = "VIEW_3D"  # 3D Viewport area (find list of values here https://docs.blender.org/api/current/bpy_types_enum_items/space_type_items.html#rna-enum-space-type-items)
    bl_region_type = "UI"  # Sidebar region (find list of values here https://docs.blender.org/api/current/bpy_types_enum_items/region_type_items.html#rna-enum-region-type-items)

    bl_category = "My Custom Panel category"  # found in the Sidebar
    bl_label = "My Custom Panel label"  # found at the top of the Panel

    def draw(self, context):
        """define the layout of the panel"""
        row = self.layout.row()
        row.operator("mesh.colorize_faces", text="Colorize Faces")


def register():
    bpy.utils.register_class(MESH_OT_colorize_faces)
    bpy.utils.register_class(VIEW3D_PT_my_custom_panel)


def unregister():
    bpy.utils.unregister_class(VIEW3D_PT_my_custom_panel)
    bpy.utils.unregister_class(MESH_OT_colorize_faces)


if __name__ == "__main__":
    register()