TAke the ryne or the scrapynges of Rubarbe, & stiepe it in white wyne, or in cleare lye: and after you haue washed your head with it, you shall weate your heares with a sponge or some other cloth, and lette them drye by the fyre, or in the Sunne: After this weate them and drye them agayne: for the oftener you dooe it, the fairer they wyll bee, without hurting your head anye thyng at all."
This is one of the few safe-sounding hair dyes we've found. If you're keen to try it, definitely use wine rather than lye! Although the lye would definitely lighten your hair, it would also damage it (besides being very dangerous to work with).
It is amusing that the recipe actually points out how safe it is... they knew quite well how harmful most of the other hair dyes were!
Source: The secretes of the reuerende Maister Alexis of Piemount, 1558