What is the purple currency

Solution 1:

The purple currency is competitive points, earned for each win in competitive matches. At the end of the season points will also be given to each player for whatever their highest rank was that they received for the season. Season 1's reward payout is listed here

3000 points are required to purchase a golden skin for a weapon. The golden skins, one per weapon, are currently the only additional skins available for purchase with these points.

Solution 2:

From the wiki,

Competitive Points are an alternative currency to Credits. One Competitive Point is earned for every match won in Competitive Play.

Competitive Points may be used to purchase cosmetic weapon skins, which apply a textured finish to weapons of standard skins/recolours and unique ones for legendary skin weapon models.