While Titleist doesn't share marketing decisions with us, there are considerations to the number of products they can put onto the shelves of pro shops.  They have 6 balls now and a total of 8 including the 2 yellow lines.  12 is likely over the top.  The tire company has 10 lines and I don't think I've seen all 10 outside of the large sporting goods companies.  Even then, they only have 2 yellow lines.

 

There are several well-responded threads about a yellow ProV1.  For hitting, I prefer the NXT Tour or ProV1x.  I'd buy which ever one was yellow.  If the new Test ball was a NXT Tour S replacement, I'd use that yellow one.  I was playing 9 yesterday to compare Test and NXT Tour - by myself.  I lose sight of drives with white balls against a gray sky or clouds.  The only ball I lost was one where I lost sight of a really sweet drive as it was still rising and I had no way to judge if it cut at the end or the approximate distance it traveled.  The others I lost sight of I found - and further than I expected!  Boomers like myself have our issues.

 

When announced at the PGA Show in January - we'll see what customer feedback has had on the move to yellow balls.