Uses git-prompt.sh (bundled with git) to show a color-coded branch name and dirty status in the prompt text if executing within a Git repository.

custom_git_prompt.sh

__colorize_arg ()
{
  local color=''
  case "$1" in
    # Other options: AM, AM/REBASE, CHERRY-PICKING, BISECTING
    *'|MERGING'* | *'|REBASE'*) # Merging or rebasing
      color="0;31" # Red
      ;;
    
    *'*+'*) # Unstaged + staged
      color="1;35" # Purple
      ;;
    
    *'*'*) # Unstaged
      color="0;33" # Yellow
      ;;
    
    *'+'*) # Staged
      color="1;36" # Cyan
      ;;
    
    *)
      color="0;32" # Green
      ;;
  esac
  
  echo -e "\e[${color}m(${1})\e[m"
}

__git_branch_or_else ()
{
  local branch="$(__git_ps1 '%s')"
  [[ ! -z "$branch" ]] && echo "$(__colorize_arg """${branch}""")" || echo "$1"
}

source /opt/local/share/git-core/git-prompt.sh
GIT_PS1_SHOWDIRTYSTATE=1
PS1='[$(__git_branch_or_else "\u@\h") \W]\$ '