I find this very complete page, with a lot of informations, which is quite interesting.
https://examine.com/supplements/curcumin/
John, that doesn't prove/mean nothing. Yellow finger is not a measured data.
Maybe taking curcumin in another form or with another additive, at only 500mg, your fingernails would become yellow. The goal of having better bioavailability is to increase curcumin's absorption rate into the blood, for the same amount taken.
You're taking 500mg but you don't know how much of that is absorbed into your blood, that's why they created plasma concentration time test, to know how much curcumin actually is in the blood.
Also, the curcumin can take a different form. Part of natural curcumin is transformed by the liver and become a conjugate form that is very poorly active. That's why some brands propose "free active curcumin" which doesn't go through the liver but directly in the blood.
But you're free to believe what you want of course, it's just your approach is not very scientific.