1. An ultrasound system, comprising: an ultrasound data acquisition unit having an ultrasound probe and configured to transmit an ultrasound signal to a target object and receive an echo signal reflected from the target object to sequentially and iteratively acquire a plurality of ultrasound data sets; and a processor connected to the ultrasound data acquisition unit, the processor being configured to form a plurality of ultrasound images including index information corresponding to a formation time of each of the ultrasound images, based on the sequentially acquired ultrasound data sets, sequentially extract feature values of a same position of the ultrasound images, allocate colors corresponding to each feature value and form a change trend image using a feature value from at least two ultrasound data sets with the colors indicative of a change trend of the extracted feature values over time.