prompt string not recognized by term or ansi-term

[Shiyuan]

Hi all,
   I am using zsh and have some special setup for the prompt string. It
works well in Terminal.app in Mac. But in Emacs term mode, something is not
right.

In my .zshrc file, I set

PROMPT="%{$fg_bold[green]%}[%d]%{$reset_color%}>"
In Emacs term mode, the color  and the directory are correctly recognized,
but there is an extra character "4m" in front,

4m[/Users/shiyuang/lisp]>

---------
Where does  the "4m" come from? It's not there in Terminal.app which is
also using the zsh. Any explanations or references ?  Thank you.

Shiyuan