I copied git-prompt.sh (https://github.com/git/git/blob/master/contrib/completion/git-prompt.sh) to ~/.git-prompt.sh
Then in my .zsrhc I have
source ~/.git-prompt.sh
setopt PROMPT_SUBST ; PS1='[%n@%m %c$(__git_ps1 " (%s)")]\$ '
The branch shows but it doesn't show color.