A method of inwardly decreasing coloring contour of color 3D object, comprising the steps of: (a) sending a 3D object to a processor; (b) performing an object slicing on the 3D object to generate a plurality of records of object print route information of a plurality of printing layers; (c) performing an image slicing on the 3D object to generate a plurality of records of color printing information of the printing layers wherein each record of the color printing information respectively describes an original coloring contour of each printing layer; wherein the original coloring contour aligns with an edge of a slicing object; wherein the number of the plurality of records of the color printing information is equal to the number of the printing layers, so that each printing layer of the 3D object corresponds to one of the color printing information and each record of the color printing information stores an original color contour of the corresponding printing layer; (d) performing an inward decrease on the plurality of records of the color printing information to generate a plurality of records of updated color printing information wherein each record of the updated color printing information describes an inwardly decreased coloring contour of each printing layer, and the inwardly decreased coloring contour is spaced from the corresponding original coloring contour by an inwardly decreased distance; wherein the inwardly decreased coloring contour is formed by spacing from the corresponding original coloring contour in a direction of inward decrease by the inwardly decreased distance and (e) causing a storage to store the records of the object print route information as a plurality of route files, and storing the records of the updated color printing information as a plurality of image files, wherein the route files and the image files are configured to be used by a 3D printer having a 3D print head and a 2D print head, and the 3D print head performs 3D printing using light transmissive materials wherein in a printing operation, the printer prints the slicing object of a corresponding printing layer based on the route file of the printing layer and then colors the slicing object based on the image file of the same printing layer. (a) sending a 3D object to a processor; (b) performing an object slicing on the 3D object to generate a plurality of records of object print route information of a plurality of printing layers; (c) performing an image slicing on the 3D object to generate a plurality of records of color printing information of the printing layers wherein each record of the color printing information respectively describes an original coloring contour of each printing layer; wherein the original coloring contour aligns with an edge of a slicing object; wherein the number of the plurality of records of the color printing information is equal to the number of the printing layers, so that each printing layer of the 3D object corresponds to one of the color printing information and each record of the color printing information stores an original color contour of the corresponding printing layer; (d) performing an inward decrease on the plurality of records of the color printing information to generate a plurality of records of updated color printing information wherein each record of the updated color printing information describes an inwardly decreased coloring contour of each printing layer, and the inwardly decreased coloring contour is spaced from the corresponding original coloring contour by an inwardly decreased distance; wherein the inwardly decreased coloring contour is formed by spacing from the corresponding original coloring contour in a direction of inward decrease by the inwardly decreased distance and (e) causing a storage to store the records of the object print route information as a plurality of route files, and storing the records of the updated color printing information as a plurality of image files,