Can you add the characters < > @, or to be able to just type some in?
You could make the colours just so you drag them in like the other markers (maybe with interface like (red fg ->) or something to show it's red foreground from here onwards.
Maybe you could categorise the items into dynamic/static/colours. eg.
Dynamic: hostname, dates, user, etc
Static: Spaces, other characters
Colours: start of a new colour, as above
Would also be good if the items from the palette never moved, so we don't have to find them each time, and can get used to the position of the items
Nice work on this. Few little suggestions:
Really nice idea though!