The thing is a popup that appears upon clicking on the input or existing colour swath or whatever. Thing contains the 23 colours as little boxes that can be clicked on, with an affordance to indicate which is currently selected. At the bottom is an input where the currently selected colour name appears, or can be typed. Some buttons (maybe just 'save') are also there.

Ideally the buttons would be at the bottom, with an x at the top, but it'll probably be the other way around because the ooui stuff is purely VE-centred.

Mockup to come, unless this turns out to be good enough. But unless someone says it is, mockup to come.

Redo layout with icon and colour displayed on the side (in js):

Actual icon select in T140165, but follows colour select workflow.

Click on colour, get popup, select colour, selected gets an outline or highlight or whatever, hit done.

Hovering over all of these should have tootips.

Who will be responsible for implementation?

Change 316719 had a related patch set uploaded (by Harej):
Color selection dialog for CollaborationHubContent theming.

https://gerrit.wikimedia.org/r/316719

Change 316719 merged by jenkins-bot:
Color selection dialog for CollaborationHubContent theming.

https://gerrit.wikimedia.org/r/316719