Way back in the thread AtLarge posted a link to the explanation that you apparently missed or didn't bother to read and which I've linked you to again below. There is in fact some reasoning for their decision. It doesn't outweigh other reasoning by a long shot, but there is in some minimal reasoning for it.
The short version is that some years back, in an effort to make the ball colors easier to distinguish on TV, Aramith changed the 4/12 ball colors to pink, and the 7/12 to brown, often known as the "TV colors".
A few years after that, Matchroom decided the orange 5 is still too hard to distinguish from other balls and so they wanted to give the 5 ball a different color (they also lightened the 6/14 to a lighter green). That was the excuse anyway, many feel they just wanted to distinguish and brand themselves by having their own trademark ball color set, and in any case that at least appears to have been a big factor. Out of all the colors left in the world to use, guess what color Aramith and Matchroom decided was the best fit to replace the orange 5/13? You guessed it, purple. Yup, the same color they previously thought was so bad it had to be removed from the set entirely.
So the obvious question then is why didn't Aramith/Matchroom just make the 4/12 purple again like it has been for all of history, and switch the pink color over to the five ball, or come up with some other new color for the five ball as this would clearly be the most logical thing to do? Aramith/Matchroom's paraphrased answer is "it is more important to us to keep the trademark of our pink four ball that we had only used for a few years, than it is for us to keep the century plus historical tradition of the purple four ball that has been exclusively used by everybody". It was the height of arrogance and disregard for the players and fans from both Aramith and Matchroom it appears.
Traditionally the object balls have the following colour code : 1 and 9 ball : yellow 2 and 10 ball : dark blue 3 and 11 ball : red 4 and 12 ball : dark purple 5 and 13 ball : orange 6 and 14 ball : dark green 7 and 15 ball : maroon 8 ball : black
www.aramith.com
My opinion is that there is no need to change any of the historical pool ball colors (especially in this day and age of great screens with accurate color representation and high definition, even on the smallest of devices), but if you feel you must, the only half acceptable change is to replace one color with a different color on any particular number ball (like to change the 7 ball color from maroon to brown), but it is never, under any circumstance ever, acceptable to move a historical color from one numbered ball to a different numbered ball (like to move the color purple from the 4 ball over to the 5 ball).
Aramith and Matchroom, if you are going to use of the color purple, you need to put it back where it has always historically been, on the four ball. Who cares about your couple years tradition of a pink four, that is not more important than everybody's century plus tradition of having the four exclusively be purple, and it is the height of arrogance gone completely astray to think otherwise. If a color has to be shifted from one ball number to another, it is the new color pink that should now be shifted from the 4 to the 5 since it has only been in marginal use for a few years, not the century plus historical purple that should be shifted from the 4 to the 5. Stop being so arrogant, illogical, and stubborn, and if you choose to continue, I may decide to stop supporting all things Aramith and Matchroom until you do.
