An example method includes enabling, by the user processing system, a user to associate a color with at least one of the plurality of pixels of a graphic, and generating a passcode. The passcode is based at least in part on the color associated with the at least one of the plurality of pixels. The method further includes transmitting, by the user processing system, the passcode to a host processing system. The method further includes determining, by the host processing system, whether the passcode matches an expected passcode that is based at least in part on a reference graphic comprising a plurality of reference pixels, each of the plurality of reference pixels having a color or a null value associated therewith. The method further includes, responsive to determining that the passcode matches the expected passcode, authorizing, by the host processing system, the user processing system to access a restricted resource.