purple, when I playtest strings, I keep them in the frame until they snap, and only use that one frame. My playtests consists of drilling with an ex-challenger level player, and hitting with a few of the juniors and pros at the Solomon Academy as well as my wife who was an ex-top 10 junior in Poland.
As soon as I string the frame, I take a ert 300, String bed deflection (SBD) on a Babolat RDC, as well as a string meter measurement. Then repeat 24 hours later, then before and after every hit until the string breaks.
I ask for feedback from the person I'm hitting with, as well as my own experience when concluding findings on a particular string.
Hope this helps.
Roger Federer Pro Staff 97 Play test video