1. An ultrasound system, comprising: an ultrasound data acquisition unit configured to transmit ultrasound signal to a target object and receive echo signal reflected from the target object to sequentially acquire a plurality of ultrasound data; and a processor connected to the ultrasound data acquisition unit, the processor being configured to sequentially extract feature values from the plurality of ultrasound data, allocate colors corresponding to each feature value and form a change trend image with the colors indicative of a change trend of the extracted feature values over time.