Here's our final image after the first render -- four spheres and two colours, grey and red. The scene only took about 10 minutes to render, but for the sake of this tutorial let's say it took 3 hours. Now we discover that we don't want the front most sphere to be red at all, we want it to have the same colour as the other spheres. We could of course change the texture and re-render the whole image, which would take another 3 hours. Or we could change the texture, delete all the other spheres and render, but since the red sphere is reflected on the big one's surface, that wouldn't be a good solution.