A method of authenticating a user of a computing device is proposed, together with computing device on which the method is implemented. In the method a modified base image is overlaid with a modified overlay image on a display and at least one of the modified base image and modified overlay image is moved by the user. Positive authentication is indicated in response to the base image reference point on the modified base image being aligned with the overlay image reference point on the modified overlay image and selection of a color for a portion of the display that matches a pre-selected color.