Yes, you can certainly paint your garage door and rolling it on is absolutely fine if you would prefer not to spray. As @EricL has mentioned, the first thing to look at is the condition of the existing surface, and if you are able to upload a couple of photos that would really help confirm the best approach. Let me know if you need a hand uploading images.
If there is any loose or flaking paint, that will need to be scraped off first, then lightly sanded to smooth and feather the edges so everything blends nicely. A good test is to run your hand across the door. If it leaves a chalky or powdery residue on your fingers, the surface will need a thorough clean. In that case, give the door a good scrub a couple of times using Sugar Soap and a Scotch-Brite pad, then rinse it well with clean water and allow it to dry completely.
You can topcoat with your chosen colour in an exterior paint, such as Dulux Weathershield. Spraying will give the smoothest finish, but a quality roller and brush will still produce a very good result if applied carefully.
Please let me know if you have any questions.
Mitchell
