All watercoloring is done with waterbased ink for the
colored parts because you have to be able to blend and move the colors with water. But for your main
outline image that you're coloring, you need to use an ink that is not waterbased, or you can emboss your main image.
My favorite ink for stamping images I'm watercoloring is India Ink in black - it's a dye ink but it is
not water soluble. There are many others too that are non-water soluble dye inks for your outlines.
Here is a link to various watercoloring techniques that may help you.