[O] org-priority-faces has no effect - how to define line colors in org-mode?

[M]

My problem is: my agenda is very long and very cluttered.
I'd like to visually emphasize all lines
A) with priority A or deadline today
B) priority B and scheduled today or before today
 

I'm using org-mode 7.8.11 with Emacs 24.1.1 and I want to change the colors
used to display agenda lines, so that e. g. all lines with priority "A" are
shown in red bold font.

I tried to change the variable org-priority-faces but it has no effect:

 org-priority-faces is a variable defined in `org-faces.el'.
 Its value is (("A" :foreground "#E01B4C" :weight bold)
 ("B" :foreground "#1739BF")
 ("#C" :foreground "#575757"))

What am I doing wrong? Could other configurations overwrite this new
setting?

I tried a lot of things, but I must be doing something wrong with the
org-priority-faces values.

Martin