Render the inside of a 3D model with a different color (not watertight)

Question

I am trying to render a 3D file and take a screenshot and it works OK. My file is not a water tight model.

1. When there is no texture (either .STL or .OBJ) and I orient (rotate) the view to look underneath, it looks black. Is there a way to assign the "inside" portion of the scan a specific color - say blue?
2. When texture is available (for .OBJ), I want to disable the texture after loading it with texture. The code below has commented out the line that does not load the texture.

My code

import open3d as o3d
import time

#Download url1 = 'https://gitlab.uni.lu/yliao/ghost_semi/-/raw/master/example/semi_io_simple/wall_right.stl'
file1 = 'wall_right.stl'
#mesh = o3d.io.read_triangle_mesh(file1, True) #Load texture if .obj
mesh = o3d.io.read_triangle_mesh(file1, False) #Do not load texture    
mesh.compute_vertex_normals()


vis = o3d.visualization.Visualizer()
vis.create_window()

vis.clear_geometries()
vis.add_geometry(mesh)

vis.poll_events()
vis.update_renderer()
time.sleep(3)
vis.destroy_window()