Using a white Sharpie Peel-Off China Marker as a resist tool. This is working really well with washes of Daniel Smith Quinacridone Gold watercolour and M. Graham Artist Black Gouache. I tried washes of India ink but found it was too intense and covered the white, even with it diluted the India Ink made things grey or obscured the white lines completely. I could use an intense black wash of the gouache with great success.
I also experimented with a white oil pastel stick which yielded great results with the watercolour and gouache. The nice thing about the China marker is that the tip can be sharpened, giving a finer line if that is desired compared to the more chubby oil pastel.
Both the China marker and the pastel stick can go on top of the dried watercolour and gouache with more washes of colour applied on top...so it has a nice "go back and forth" process to it.
As well I used graphite in a stick form and found it also resists the watercolour and gouache.