What I mean by “on your computer” is not that it originates on your computer, but that some form of it exists there–namely this is going to be images, text, links, etc that the ad company hosts and a website will normally download temporarily along with the rest of the site’s content. Once your computer has that site’s information you can do anything you want with it. Importantly what exists on your computer is a local copy of what the ad servers host. If you decide to color ads blue on your computer that only affects your copy. The original ad, and everyone else’s copies remain intact.