My neighbour set up an indigo vat this weekend, and invited me to come and dip bits of fabric into it. It was fascinating to watch the fabric turn from yellow to green to blue as it oxidized.
We ran out of things to dye, and started on anything that wasn't nailed down or already blue. One of the exciting discoveries was that printed bits retained their colour - red stayed bright red. Of course nothing in my scrap box is safe now.
(As you can see in this pic, white cat fur appears to be completely impervious to indigo. What a surprise. )