I always add something like this to my /root/.bashrc
#Make prompt red when logged in as root
...which also adds user name and path to the prompt...
Also, if you do not have a /root/.bash_profile, you will need to create it and add these lines:
# Get the aliases and functions
if [ -f ~/.bashrc ]; then
See man bash, INVOCATION for the precedence order of config files and how they are loaded.