Finally, a foolproof dyestuff that really was proof against me!
Collected about a dozen walnuts from the neighbor's yard (our young black walnut didn't like this spring's weather and dropped all its nuts on the ground before the end of June). Husked them and laid out most of the bits to dry, but saved one walnut's worth (93g fresh) and dropped the bits in ~1 gallon of water (which turned dark yellow instantly).
24 hours later, added 1/2 c white vinegar (recommended in J.N. Liles' book).
48 hours after dyepot started, simmered 1 hour and strained. Added 6g buff-colored wool (previously dyed with either mulberry or buckthorn) and 6g alum-mordanted undyed wool to the warm dyebath.
72 hours after adding wool, removed and rinsed it. Can't tell which wool was predyed, but both bits have a very noticeable green tinge!
Added 25g more of the predyed wool, tied in various places with sock yarn, to see if I could get patterns of light/dark dye. Simmered for 2 hours, left to soak 24 more hours, then rinsed and set out to dry. Looks like there are definitely dark/light spots, but it looks more like the dyestuff had settled to the bottom of the pot; the parts touching the bottom are very dark brown, while parts floating just below the surface are light. Guess that teaches me a lesson about remembering to stir the pot once I'm done simmering, not just let it sit there untouched for 24 hours!
Still no pictures, haven't worked out where to host my photos now that Flickr's complaining about reaching 200...