In GB Studio there are several features that only work correctly on Color systems, not the original DMG systems. Most projects will still be fine to run on both, but there are several reasons why you might want to limit the game to only run on Color systems, such as if the game you've made uses features that don't work correctly without the additional power of those systems.
In order to create a lock-out screen in these instances, you would use the function 'Is Color Mode Available' as a check on starting the game. If false, then you direct to a scene that in the background simply states the game only works on Color systems, and if true, you direct to the actual title screen of the game.
If you're not sure whether you need a lockout screen, give your game a quick test on a DMG system or emulator in DMG mode and see it if still works as you'd like. If it doesn't, then you can look into making the lock-out screen.