I'd polled the crafty types on said mailing list a couple months ago, asking whether anyone had tried dyeing with madder on a naturally darker fibre to make a "truer" red, and got a vague consensus that the result would be a grey-red, or something similar.
Actually tried that theory out, today. I was doing a dyepot of approximately equal weights fibre and dyestuff in the tap water of our new home (much, much softer than our old place!) and tossed in natural white wool from a couple different breeds, some bleached linen, and at the last minute, a sample of grey Gotland wool. It was a one-hour dye liquor, two hour dye bath kept between 60 and 70ºC, alum and cream of tartar mordant on everything but the Gotland, as it was last-minute.
Results justaposed with original colours:
Anti-clockwards, from the spindle: blue-faced Leicester wool: natural and dyed; Corriedale wool: natural and dyed; Gotland wool: natural grey and dyed; linen: bleached and dyed.
The linen turned a lovely pale pink, like I'd thought. The natural whites went the normal madder russet. The grey turned brown, though, but brown from a distance. It seems like a black-walnut-hull brown in the photo, but up close it's very definitely a grey with orange fluff. It looks kind of sickly, and dare I say, faecal. I am, to be quite honest, not that impressed; shall probably overdye it with indigo to black it up, once I get up the gumption to try messing with scarier chemicals again. Don't think I'll try madder over grey again.
Along those lines, when you're over-dyeing madder with indigo to attempt a purple, ought it to be tried with a less orangey red than what I have from today? I'm worried about getting another sickly brown. :P