I don't remember creating one and I'm just trying to print in black and white.
Normally you don't need to enter a root username and password to use CUPS. The error is likely to be in /etc/cups/printers.conf You could check the authorization entries in your printers.conf
Even simpler though is that CUPS.org recommends just using your normal admin username and password when asked for authorization:
Use the username and password for your computer.
You will need an account with admin privileges: most OS X computer owners have such an account, it's usually only in an lab or a company where you wouldn't have admin privileges.
I hope this works in your case, Esmeralda. If it doesn't please post some follow up information, such as your printers.conf file.