At the bottom of the macOS standard color picker it seems there is an area to put “favorites”?
But if so, how do you add them, and are they “system” wide or application specific, or?
Drag the little square of your current selection to the favorites, and they were system wide the last time I looked, but that was 10.11
(one of those handy little Mac things 
)
Cool… thanks… I looked all over the web, and nobody really said
Where is the macOS Users Guide ?
(lost along the years, probably before mac OS get an ending X)
Well, those colors are stored in a NSColorListMBS object.
And with NSColorPanelMBS you can attach a NSColorListMBS to the panel to provide your favorites and store them in your preferences.