Wow! Thanks, I thought this was not possible, but then I found this: http://www.emacswiki.org/emacs/FacesPerBuffer
Just look at the wiki example, it looks like you need:
(make-face 'php-comment-face)
(set-face-foreground 'php-comment-face "LightGrey")
(add-hook 'php-mode-hook
(lambda ()
;; ...
(set (make-local-variable 'font-lock-comment-face)
'php-comment-face)
;; ...
: Emacs defaut font face per-buffer/mode
UPD
cc-mode, (add-hook csharp-mode-hook ... (add-hook c-mode-hook ..., :
(make-face 'c-comment-face)
(set-face-foreground 'c-comment-face "Red")
(add-hook 'c-mode-hook
(lambda ()
;; ...
(set (make-local-variable 'font-lock-comment-face)
'c-comment-face)))
(make-face 'cs-comment-face)
(set-face-foreground 'cs-comment-face "Blue")
(add-hook 'csharp-mode-hook
(lambda ()
;; ...
(set (make-local-variable 'font-lock-comment-face)
'cs-comment-face)))
, csharp c-mode. (remove-hook ... .