Where can I find a source for your first point? This site[1] mentions this test as a way to detect adulteration by artificial coloring in turmeric powder, not adulteration by lead.

I think the point (at least the one I gathered from the article) is that turmeric is sometimes artificially colored to make it the right shade of yellow, and that lead is in the chemical often used for that coloring. Thus, evidence of artificial coloring is a heuristic for presence of lead.

I suppose your turmeric could fail the test, and you could decide that the failure is not a sufficient proof of lead presence, because it might be a different lead-free artificial coloring that caused the test failure. That is absolutely your right to decide.

Wouldn’t lead contamination cause the turmeric to sink?

Lead Chromate, not Lead.

But apparently it's also not very soluble in water, so the question about sinking stands.

Possibly, a chemical powder is much finer than a ground-up plant matter, and takes longer to settle.

https://en.wikipedia.org/wiki/Lead(II)_chromate#Safety_hazar...

I've actually had a successful test with the water method. The lead coloring shows immediately and remains dissolved or in suspension or whatever for days, possibly forever. Turmeric doesn't usually color water at all.

There are youtube videos showing how to do the turmeric test with water. Definitely, everyone that uses it, should know this.

https://www.youtube.com/watch?v=tXWPf0HQd5U

(Testing Turmeric Powder adulteration with Artificial Color)

I just tried this test with some turmeric powder I had at home and it failed according to what is shown in the video. I happened to have a lead testing kit, and it doesn't have lead though. The powder does have some ginger and other herbs in it, so apparently harmless adulterants other than lead can cause it to fail this visual test.

Have you checked that test with known lead chromate to rule out test error?