A color- or grayscale-sensing, magnetic, mobile, marking robot, comprising: a robot body including a top surface and a bottom surface and the robot body defining a retainer; a magnet in or coupled with the robot body; at least two wheels coupled with the robot body and extending from the bottom surface of the body, allowing the robot to climb and be displaced across a vertically oriented surface of a magnetic structure with the wheels in contact with the vertically oriented surface, wherein the retainer is centered between the wheels; an actuator configured to reciprocally displace a shaft in the retainer, wherein, (a) the shaft takes the form of a marker including a marking tip, wherein the retainer is an aperture defined by the robot body and through which the shaft is reciprocated, wherein the shaft fits in the aperture of the robot body and is actuatable by the actuator to reciprocally place the marking tip in contact with the vertically oriented surface and remove the marking tip from contact with the vertically oriented surface, or (b) the shaft includes or is coupled with an eraser, wherein the shaft fits in the retainer of the robot body and is actuatable by the actuator to reciprocally place the eraser in contact with the vertically oriented surface and remove the eraser from contact with the vertically oriented surface; an array of color or grayscale sensors on at least one of the top surface and the bottom surface and configured to detect color or grayscale patterns as the robot is displaced across the vertically oriented surface; and a computing device in communication with the sensors, the actuator, and the wheels, wherein the computing device includes a processor and computer-readable memory in communication with the processor, wherein the computer-readable memory includes non-transitory program code for at least one of the following actions: a) displacing the robot; and b) actuating the actuator in response to and as a function of measurements from the sensors. a robot body including a top surface and a bottom surface and the robot body defining a retainer; a magnet in or coupled with the robot body; at least two wheels coupled with the robot body and extending from the bottom surface of the body, allowing the robot to climb and be displaced across a vertically oriented surface of a magnetic structure with the wheels in contact with the vertically oriented surface, wherein the retainer is centered between the wheels; an actuator configured to reciprocally displace a shaft in the retainer, wherein, (a) the shaft takes the form of a marker including a marking tip, wherein the retainer is an aperture defined by the robot body and through which the shaft is reciprocated, wherein the shaft fits in the aperture of the robot body and is actuatable by the actuator to reciprocally place the marking tip in contact with the vertically oriented surface and remove the marking tip from contact with the vertically oriented surface, or (b) the shaft includes or is coupled with an eraser, wherein the shaft fits in the retainer of the robot body and is actuatable by the actuator to reciprocally place the eraser in contact with the vertically oriented surface and remove the eraser from contact with the vertically oriented surface; (a) the shaft takes the form of a marker including a marking tip, wherein the retainer is an aperture defined by the robot body and through which the shaft is reciprocated, wherein the shaft fits in the aperture of the robot body and is actuatable by the actuator to reciprocally place the marking tip in contact with the vertically oriented surface and remove the marking tip from contact with the vertically oriented surface, or (b) the shaft includes or is coupled with an eraser, wherein the shaft fits in the retainer of the robot body and is actuatable by the actuator to reciprocally place the eraser in contact with the vertically oriented surface and remove the eraser from contact with the vertically oriented surface; an array of color or grayscale sensors on at least one of the top surface and the bottom surface and configured to detect color or grayscale patterns as the robot is displaced across the vertically oriented surface; and a computing device in communication with the sensors, the actuator, and the wheels, wherein the computing device includes a processor and computer-readable memory in communication with the processor, wherein the computer-readable memory includes non-transitory program code for at least one of the following actions: a) displacing the robot; and b) actuating the actuator in response to and as a function of measurements from the sensors. a) displacing the robot; and b) actuating the actuator in response to and as a function of measurements from the sensors.