Many thanks to all of you for your help!!!!
For those who want to know:
Customizing the prompt for root (with a red prompt) is:
set prompt="%{\e[1;31m%}@%m:%~# %{\e[m%}"
As you might notice for csh you have to replace the \[ \] with %{ %}
> >
> > $ export PS1="\[\e[1;31m\]@%m:%~# \[\e[m\]"