A system for informing an operator of a work vehicle where an actual travel track deviates from a target travel route, the system comprising: a display configured to display the target travel route of the work vehicle in a work field; and a control device configured to: calculate an index value indicating a degree of a deviation of the actual travel track from the target travel route; display the index value on the display in any of a plurality of colors including a first color and a second color, in accordance with a magnitude of the index value; characterized in that the control device is further configured to: color-code, on the display and in accordance with the index value, a portion of the target travel route where the work vehicle has traveled; wherein the control device is further configured to color-code: in the first color, a first section of the portion of the target travel route where the work vehicle has traveled, wherein, in the first section, the index value has been displayed in the first color; and in the second color, a second section of the portion of the target travel route where the work vehicle has traveled, wherein, in the second section, the index value has been displayed in the second color. a display configured to display the target travel route of the work vehicle in a work field; and a control device configured to: calculate an index value indicating a degree of a deviation of the actual travel track from the target travel route; display the index value on the display in any of a plurality of colors including a first color and a second color, in accordance with a magnitude of the index value; calculate an index value indicating a degree of a deviation of the actual travel track from the target travel route; display the index value on the display in any of a plurality of colors including a first color and a second color, in accordance with a magnitude of the index value; characterized in that the control device is further configured to: color-code, on the display and in accordance with the index value, a portion of the target travel route where the work vehicle has traveled; wherein the control device is further configured to color-code: in the first color, a first section of the portion of the target travel route where the work vehicle has traveled, wherein, in the first section, the index value has been displayed in the first color; and in the second color, a second section of the portion of the target travel route where the work vehicle has traveled, wherein, in the second section, the index value has been displayed in the second color. color-code, on the display and in accordance with the index value, a portion of the target travel route where the work vehicle has traveled; wherein the control device is further configured to color-code: in the first color, a first section of the portion of the target travel route where the work vehicle has traveled, wherein, in the first section, the index value has been displayed in the first color; and in the second color, a second section of the portion of the target travel route where the work vehicle has traveled, wherein, in the second section, the index value has been displayed in the second color. in the first color, a first section of the portion of the target travel route where the work vehicle has traveled, wherein, in the first section, the index value has been displayed in the first color; and in the second color, a second section of the portion of the target travel route where the work vehicle has traveled, wherein, in the second section, the index value has been displayed in the second color.