In the color stops popover the value input in the popover is controlled by the color stop component.

However, in some cases it would be nice to be able to change the appearance in some ways, like defining prepend and append, like in these mockups of the Lens color palette builder:

API Suggestion

Add

inputProps?: Partial<EuiInputProps>;

To the properties of the color stops component - if these are set, they are used as the default before the specific props are set in https://github.com/elastic/eui/blob/master/src/components/color_picker/color_stops/color_stop_thumb.tsx#L342 - this means props like readOnly, value and onChange are not configurable this way.