I nearly gave up on this app before realising how good it is. It is not immediately apparent (or was not to me) how to use it. It allows you to create or edit palettes of colours that are then synced directly with those in the system color picker.
There are only two windows you need to worry about, they are both a little weird, and their interaction is not well documented, but once you understand the whole thing makes sense.
The square main editing window is where all the work is done; the Palettes window is just responsible for import/export and sync. (There is also a Palette Generator window with six basic options for generating palettes based on the currently selected colour, but this is an optional feature.)
The main editing window gives a much better interface than the system picker for editing and comparing colours. One oddity is that palettes cannot be saved from this window: you have to switch to the Palettes window and press + to save the current contents of the editing window to a new palette —and instantly sync to the system picker. Unlike some similar apps, colours are saved in the Generic RGB colour space not downsampled to sRGB, so values are preserved.
There is no Undo, but there is a comprehensive history of all edits, which is arguably better.
ColorWell seems to me virtually unique in what it does, and to do it very well. There is one issue: a bug occasionally disables direct editing via the colour values in the editing window, but this can be cured with a relaunch.