I do that in bash for root/non-root users (root is red, non-root is blue):

    if [[ ${EUID} == 0 ]] ; then
        PS1='\[\033[01m\][ \[\033[01;31m\]\u@\h \[\033[00m\]\[\033[01m\]] \[\033[01;32m\]\w\[\033[00m\]\n\[\033[01;31m\]\$\[\033[00m\]> '
    else
        PS1='\[\033[01m\][ \[\033[01;34m\]\u@\h \[\033[00m\]\[\033[01m\]] \[\033[01;32m\]\w\[\033[00m\]\n\[\033[01;34m\]\$\[\033[00m\]> '
    fi

Interesting to think about using it across servers, though.