From what I have read up on all epoxies yellow to a degree but some more than other ....and as you said white enhances it..
As I said before I really like Bullards D2...But would like to test out the ProKote as well ...seem to yellow less
At the Moment Billy Vivona is doing a test on facebook with ProKote and thread Master and other mixes..
He has made up samples ...and put tinfoil over half of the sample and leaving them for awhile in the sunlight ..so down the track a bit we will see the results