I've been a spotting fool lately and am loving Sumi. Very fast once you get the brush loaded just right.
As far as tone matching, I tend to print on colder, neutral papers and, in many years, I have never had a problem. The fellow who turned me on to it used Elon to adjust contrast on graded papers which can add a pronounced greenish tone and he used it all the time with great success (although if he was spotting a sepia print he used Kodak red opaque). This led me to the conclusion that the importance of tone matching might be over-rated. At least with ink. I am assuming that with warm enough papers, it must start to become noticable and that water color idea sounds interesting.