A lot of apps have a custom way of dealing with colors, just like Affinity Designer. But most of those app also support the native OS X color picker, c.f. by giving a simple shortcut in the menu like "Show Colors" to open it.
The color picker is an app that's been around for years. Many of us have vast collections of palettes for our projects in this app. With apps like Hues and Colors Pro plus the huge amount of plugins which is available for it, it's still happen to be one of the best ways to manage colors and get in them in our designs.
In short: In my eyes this is a mandatory addition.