I've worked out how to do this, with luck it works for what you're trying to do.
Here is an example:
(defun color-text-field (ink)
(let ((pane (find-pane-named *application-frame* 'text-field)))
;; The Goatee screen area keeps track of its own foreground color.
(setf (goatee::foreground-ink (climi::area pane)) ink)
;; The cursor is still drawn in foreground ink of the sheet-medium.
(setf (medium-foreground pane) ink)
;; Now force repaint of the sheet
(repaint-sheet pane (sheet-region pane))))
(defun red-text-field (gadget)
(color-text-field +red+))
(defun green-text-field (gadget)
(color-text-field +green+))
(define-application-frame colored-text-editing ()
()
(:panes
(text-field :text-field)
(red-button :push-button :activate-callback 'red-text-field :label "Redify")
(green-button :push-button :activate-callback 'green-text-field :label "Greenicate"))
(:layouts
(default
(vertically ()
text-field
(horizontally ()
red-button
green-button)))))