I receive the requirements from the developers and start doing some benchmark and define our users needs. The picker doesn't have to offer YMCK modifers for example, but being able to use an HEX code is important when they're customising the theme to fit their brand colour exactly.
So I cleared up the dev requirements thinking of the MVP.
Below are the wireframes that show the interactions possible. For example having a palette only picker when the user shouldn't be able to add a custom colour. How to select a colour, how to add one to the custom palette, how to delete it.
Then I defined the keyboard interactions for the devs to implement them.
As well as the touch interactions.
Finally, I created a quick Invision prototype and clicked-through while recording my screen to explain the interactions to the devs better.