Oh, just for convenience. It must be inconvenient to use watercolor gouache when setting up the renderings. Mosaic pen. Colored lead is a dry hard pen, which is convenient to use. In particular, it is difficult to draw lines such as straight lines. For watercolor gouache, it can be painted very carefully and easily controlled. Of course, you can also use watercolor gouache to draw renderings. It's just sometimes inconvenient.