It can be quite dangerous when trying to test in the preview or custom sandboxes if you also need to have production open as the colour schemes are the same, so it's easy to make a change in production by mistake. It's also more time consuming having to check the URL of the environment you are in or scroll up to the top of the page to see the banner stating if you are in preview/sandbox. A simple change of scheme or colour would make it so much clearer that you are in a test environment.
