From the Natural Dyes list on Yahoo!Groups:

Every so often I actually read the text in books I get mainly for the illustrations — this is one: *William Morris: Decor and Design* by Elizabeth Wilhide (ISBN 0-8109-3623- 2; Abrams, 1991).
I hadn’t realized that he used only natural dyes for all of his
interior fabrics, rugs, upholstery.. . yards and yards and hundreds of
yards of printed cottons and other fabrics made of linen, wool, silk,
mohair for upholstery, drapes and curtains, rugs, tapestries.. .

Lightfastness was important, and the aniline dyes of the time (late
1800s) were "prone to fading, discolouration, and bleeding." When
natural dyes "mellowed," they didn’t upset the "original tonal balance.
… Fabric dyed with indigo … kept its colour by candlelight. "

The dyes he used: indigo, madder, cochineal, kermes, weld, walnut roots, and some local dyes like heather, birch twigs.