I thought I'd add my experiences to this thread (which, by the way, has some very good info in it). I decided to have a go at making walnut dye and did a lot of reading. I soaked the husks for a while, then boiled them and let them soak a while longer. After straining through some calico (very time consuming) I ended up with a nice dark liquid. My first effort at dying leather wasn't that successful. I soaked a piece for a couple of hours and let it drain dry. The leather hadn't changed colour much, although there was more darkening just along the edges. I might have to let some of the liquid evaporate to thicken it up a bit.
Anyhow, I've decided to have another go with this year's crop - the birds (cockatoos) start to get into the nuts while they're still green so I'm not going to get an edible crop (again) so I might as well make some use of them. This time I'll try something different that I read about. I started off with the complete nuts soaking in a large bucket, soon there was a second bucket, then two more buckets full of nuts, with more to come, so I used a largish wheelie bin, with a lid, and dumped them all in it and as the birds drop more nuts I just throw them in whole. Being Summer (and bloody hot!) the bin is in the sun and heats up the water/nuts inside which should speed up the fermentation. Now for the good bit, an easy way to hopefully make the dye - I'll just leave the whole lot to stew for at least a year! One chap posted that he was doing the boiling/simmering thing but also had a heap sitting in a barrel outside and he found that after a year or so it produced a dye that was no different to the stuff he'd been boiling/simmering/peeling.
It also means that if, for some reason, the dye doesn't work I haven't wasted lot of time (and fuel) making it.