I stand corrected — not only are there henna dyes that work on light-colored hair, but some are on sale right now.
Remediesdirect.com is offering Herbal Choice Mari henna dye in Apricot Gold for half price: $7.95 for two ounces of powder. The liquid version, recommended for grey hair, is also on sale. The site says this color will work on blonde, grey, or light brown hair; see their color chart. Unlike many henna products, this is made in the U.S.A.
Any order over $7 qualifies for a discount if you order online — just click on Today’s Coupons. For orders over $50, shipping is free in the U.S.A.