How to set minibuffer-prompt face foreground conditionally?

[Richard Cobbe <cobbe@ccs.neu.edu>]

I'm using GNU Emacs 23.3.1 on MacOS 10.6, although I've observed the same
behavior with 23.1.1 on Ubuntu 10.10.

I'm having trouble setting the foreground color of minibuffer-prompt in my
.emacs file.  If I evaluate (set-face-foreground 'minibuffer-prompt "cyan")
either with M-x eval-expression or in the scratch buffer, then the face
changes color.  But if I put the same thing into .emacs and restart, it has
no effect at all.  Customizing the font works, but this isn't sufficient
for what I'm trying to do; see below.

Context: I want to change the foreground color of the minibuffer-prompt
face, but only when window-system is nil.  Normally (when window-system is
one of 'mac 'ns or 'x) the minibuffer prompt is readable, but when I run
emacs in a terminal, the minibuffer prompt shows up as dark blue on the
only-slightly-different dark blue that I use for my xterm background.  I do
have '(background-mode . dark) in default-frame-alist for all
window-systems.

Is there a way to do this that I'm overlooking?

Thanks,

Richard