The obvious coloring agents to use in this case would be vegetable colors such as henna or chamomile. These have many disadvantages, though. The colors they produce are not natural-looking and they fade very quickly, leading to more frequent applications, unnatural colors and, ultimately, drier, brittle hair. Also, in a similar way to semi-permanent colors, there is a continual overlap in application, leading to an unnatural appearance -- particularly on the ends.