Comment by blincoln

https://dynomight.net/img/colors/generate.html?inside=0000ff... is the one I was using.

https://dynomight.net/img/colors/generate.html?inside=0000ff... was suggested by another commenter.

But as I mentioned in a parallel thread, I think the effect is caused by an imperfect optical path. Without my contacts in, there's no purple ring.