1. An embroidery data generating device which generates embroidery data usable to sew, by a sewing machine, an embroidery pattern composed of a plurality of color-based pattern portions, the device comprising: an embroidery data storage unit which is configured to store a plurality of embroidery data; an embroidery data selection unit which is configured to select a desired one of the embroidery data stored on the embroidery data storage unit; a color data storage unit which is configured to store data of a plurality of defined colors; and an assignment unit which is configured to randomly extract colors from the colors stored on the color data storage unit, the extracted colors being used as thread color data for specifying colors of the color-based pattern portions respectively, the assignment unit assigning the extracted colors to the respective color-based pattern portions composing the selected embroidery pattern, wherein when the embroidery data selected by the embroidery data selection unit includes thread color data of two or more color-based pattern portions having a same color, a randomly extracted color is assigned to said two or more color-based pattern portions so that said two or more color-based pattern portions have the same color and the thread color is updated.