Apparatus and method for monitoring a system in which a fluid flows and which is characterized by a change in the system with time in space. A preselected place in the system is monitored to collect data at two or more time points correlated to a system event. The data is indicative of a system parameter that varies with time as a function of at least two variables related to system wash-in and wash-out behavior. A calibration map is made on a calculated basis with each pixel or voxel representative of a color hue indicative of wash-out behavior and a color intensity indicative of wash-in behavior. The calibration map serves as a criteria for selecting the time points. Software and a data processing system are provided to develop a color coded output map. The calibration map, the color coded output map and image of the preselected place are also novel implementations.