I use the brand Pebeo, for both black and white ink (in bottle) and Uni-ball Signo for the pen.
Oh, I remember using white ink with brush, and tried to paint small waves in the water, but it didn't look well, I erased with water, and indeed il mixed weirdly with the black layer. It is necessary to wait the under layer is very dry.. but even this way, white ink turns easily grey-ish.
For this reason precisely I don't like this painting much. So thank you for the kind words, actually I was quite surprised that so many people seem to like it.