If the following lines are not present in ~/.emacs, we suggest they are added, so that the font-lock mode (syntax coloring support) is enabled for all major modes in Emacs that support it.
(global-font-lock-mode t) ; GNU Emacs
(setq font-lock-auto-fontify t) ; XEmacs
(setq font-lock-maximum-decoration t)
These settings and more are also available through M-x customize-group <RET> font-lock.
If one wants to add font-locking only to the prolog mode, the two lines above could be replaced by:
(add-hook 'prolog-mode-hook 'turn-on-font-lock)
Similarly, to turn it off only for prolog mode use:
(add-hook 'prolog-mode-hook 'turn-off-font-lock)