The red color of seal ink comes from the mineral cinnabar, which is then made into the color known as vermillion. Mercury also comes from the same mineral (remember the bright red color of a thermometer?) and since mercury, which is highly poisonous, can be absorbed through the skin, I recommend avoiding as much as possible any skin contact with seal ink. Someone once told me that seal ink is safe, but I prefer to err on the side of caution. So don’t eat it!
A Japanese seal carver told me a story about her student days when she had washed red seal ink into the school sink. Later in class, health inspectors came to report there were dangerously elevated levels of mercury found in the school’s waste water. She didn’t confess, but also didn’t wash seal ink into the sink again. Japanese seal artists use red ink during the design process, grinding a red ink stick onto a small ink stone used just for that purpose, and then, with a small brush, painting the design in red. These stones are not washed off (which she learned the hard way), but only wiped off. Chinese seal artists generally don’t use this method of using red ink and so don’t have this problem.