When I was dyeing with Japanese indigo last summer I pre-mordanted one roving and some silk with alum so that I could experiment with over dyeing. I decided to raid the stash of frozen flowers I have stuffed in my freezer and use them to over dye the blue wool and silk and also some wool and silk which were a nondescript beige (I've forgotten what they were dyed with).
When I opened a bag of frozen French marigolds - oh my! - the scent of a summer garden. The green fibre and fabric is the over dyed Japanese indigo, the old gold is the over dyed beige and the yellow (actually brighter than shown) is white wool fabric.