The black Kuretake Manga ink is not only waterproof, but stands up well to alcohol based markers and other water based inks as well. We did our own test here at JetPens using a water brush, Copic Sketch Marker - Yellow Green, and a Uni-ball Signo DX UM-151 Gel Ink Pen - 0.5 mm - Orange.

As long as it is able to dry completely, the Kuretake Manga Ink is completely waterproof, and will work beautifully with watercolor washes and other water-based inks.

It also performed wonderfully to the Yellow Green Copic Sketch Marker we used. Remember, it WILL smudge if you draw on it with these markers before the Kuretake ink is fully dry.

Here is our final recap: