Setting the color of tabs on a browser isn't something a website can do, but websites can specify a theme color that some browsers can choose to use. I know Chrome on mobile does this, but it gets overridden if the user turns on dark mode.
Maybe another solution could be to have different colored tab icons that are just randomized on every page load. Idk if there are caching issues where the browser would just remember the icon for a given domain and not take into account the different icon urls every page load.
(post is archived)