From the article:
Francl said that if people want to color their hair in the most natural way possible, they should use henna, mineral powders or lemon juice along with sun exposure.
“To take the color out of your hair and put new color on to cover up the color is a nasty chemical process,” she said.
Francl has been an outspoken critic of the pseudo-science and marketing-speak in which “natural” is always good and “chemical” always bad. Media outlets she's appeared in include Slate, The New York Times Magazine, and NPR's On the Media.