Hi cookieman,
I think that your colors or enviroment problems doesn't depending on "-ro" option.
However, if you'll never open your maps by user root, you can change your map's permission with "ovwchmod" and "ovwchown" command.
So, if you set
ovwchmod 444 <map name>
ovwchown root:<group> <map name>
your maps will can be open by all user in read only mode and in read/write mode only by user root.
Regards