There are a few customization options that are specific to the Nextstep port. For example, they affect things such as the modifier keys and the fullscreen behavior. To see all such options, use M-x customize-group <RET> ns <RET>.
The standard Mac / GNUstep font and color panels are accessible via Lisp commands. The Font Panel may be accessed with M-x ns-popup-font-panel. It will set the default font in the frame most recently used or clicked on.
You can bring up a color panel with M-x ns-popup-color-panel and drag the color you want over the Emacs face you want to change. Normal dragging will alter the foreground color. Shift dragging will alter the background color. To discard the settings, create a new frame and close the altered one.
Useful in this context is the listing of all faces obtained by M-x list-faces-display.
In Mac OS X 10.5 and later, Emacs uses a Core Text based font backend by default. If you prefer the older font style, enter the following at the command-line before starting Emacs:
% defaults write org.gnu.Emacs FontBackend ns