[Bug-AUCTeX] fon-lock messes up

[Baranas]

Hello, I am using Emacs+Auctex-11.86  and I need additional highlighting schemes
for my latex documents. However if I create my own face and add 

  (add-hook 'TeX-mode-hook
    (lambda ()
        (font-lock-add-keywords nil
            '(("\\\\alpha" 0 my-face t)))))

to my .emacs  file, font-lock after each occurrence of \alpha switches off for
some part of the text. However if i use  predefined face like
font-lock-warning-face it works (unfortunately not in subscripts). I am looking
for a way to highlight some commands in math-mode.