Sometime after version 10.7 of the game, you'll receive a call from publicist Kalinda King.
Kalinda will re-introduce herself, and then tell you about a new pop-up shop that has opened in Santa Monica.
It's called the Dye Exchange, and the shop allows players to exchange dyes they have on hand for other dyes.
You can choose to visit the shop right away, after which the screen will turn black and your character will be in the shop, and the goal will be completed.
You can also choose to visit it later. In that case, travel to Santa Monica for 40, swipe to the right, and tap the purple icon on the first building.
Select "Dye Exchange" from the drop-down menu to enter the shop.
There, you'll meet three characters with whom you'll be able to exchange dyes.
The exchange rates are as follows: