Hi Marc and Wayne,
I've used water based colors for years to highlight scratches and rough spots. The color does disappear when ferric nitrate is applied but most of the color is scraped off before. Sometimes I use pure black particularly on cherry and walnut. After scraping, it is still embedded in the grain and gives the final color an old mellow look. However, I sometimes I add layers of color. I test scraps of the maple with just ferric nitrate (aqua fortis) and sometimes the resulting colors are very monochromatic, meaning the figure highlights are just darker shades of the same red-brown color. Depending on my objectives, I may layer other colors like yellow, orange, and red, to gain more diversity and interest in the color of the wood.
dave