A silly example of coloring sentences/paragraphs for use with multilingual, aligned texts.

aligned-text-overlays.el

(defun brian-rotate (list)
  (append (cdr list) (list (car list))))

(defvar-local brian-stupid-buf-overlays nil)

(defun brian-color-remove-overlays ()
  (interactive)
  (while (not (null brian-stupid-buf-overlays))
    (delete-overlay (pop brian-stupid-buf-overlays))))

(defun brian-color-next-paragraph (color)
  (let ((startpt (point))
	endpt this-overlay)
    (save-excursion
      (when (eq (forward-paragraph) 0)
	(setq endpt (point))
	(setq this-overlay (make-overlay startpt endpt))
	(overlay-put this-overlay 'face `(:background ,color))
	(setq brian-stupid-buf-overlays 
	      (add-to-list 'brian-stupid-buf-overlays this-overlay)))
      endpt)))

(defun brian-color-next-sentence (color limit)
  (let (startpt this-overlay endpt)
    (when (< (forward-sentence) limit)
      (setq endpt (point)
	    startpt (backward-sentence))
      (forward-sentence)
      (setq this-overlay (make-overlay startpt endpt))
      (overlay-put this-overlay 'face `(:background ,color :weight bold))
      (setq brian-stupid-buf-overlays 
	    (add-to-list 'brian-stupid-buf-overlays this-overlay)))
    endpt))

(defun brian-stupid-colors ()
  (interactive)
  (let ((colors '("lightgreen" "yellow"))
	(par-colors '("white"))
	this-overlay parend)

    (brian-color-remove-overlays)

    (save-excursion
      (goto-char (point-min))
      (while (setq parend (brian-color-next-paragraph (car par-colors)))
	(let ((local-colors colors))
	  (while (and parend 
		      (< (or (brian-color-next-sentence (car local-colors)
							parend)
			     (point-max)) parend))
	    (setq local-colors (brian-rotate local-colors)))
	  (setq par-colors (brian-rotate par-colors)))
	(goto-char parend)))))