For a freehand line image drawn onto a PC screen where a user-selected reference image, e.g., a color photograph, as a model is faintly displayed with low contrast, we proposed a method for automatic coloring with a constrained Delaunay triangulation that divides the image into small triangles. Using a prototype system based on the proposed method, users can complete impressive pictures by only drawing lines. Our coloring method begins with the triangulation for the set of sampling points on the drawn lines, followed by sampling of color in each triangle on the reference image, smoothing of color among neighboring triangles, and painting of each triangle with the smoothed color. The result of the triangulation is modified such that it satisfies a constraint where its divided lines should not cross over the drawn lines not to mix colors beyond the drawn line. Our prototype system can display the coloring result of the current drawings immediately for convenience. So, the user can check the effect of a newly drawn line on coloring at any time. As the result of the coloring depends on how the user draws freehand lines, it can be seen as an art work with the individuality of each user’s drawings.