1. An image-based motion sensor, comprising: a camera system, arranged for generating an image output including a plurality of captured images; and a processing system, arranged for obtaining a motion sensor output by processing the image output, and identifying a user input as one of a plurality of pre-defined user actions according to the motion sensor output, wherein different functions of at least one application performed by one electronic device are controlled by the pre-defined user actions; and the motion sensor output includes information indicative of at least one of a motion status and an orientation status of the image-based motion sensor; wherein each of the captured images comprises more than one color component, and only values of one single color component are involved in obtaining the motion sensor output; wherein the camera system performs predetermined processing upon the captured images to generate grayscale images, and only the values of said one single color component derived from the grayscale images are involved in obtaining the motion sensor output.