I have had this done to a phone i had some time ago, it is a loose cable somewhere along to the screen and the “random colors” is normal for some devices of age, it is a part of the screen that isn’t getting told what color to be (to some extent) so it turns on or off some pixels because it ist getting told to have those parts of the screen to be “off”, it will not cause any more damage, so feel free to get a new screen but it’s not required and will still function normally without issue.