The maximum length of a string sent between a Torque server and client is 255 bytes, because length is represented as a single unsigned byte.  Because Torque represents characters as strictly byte-for-byte, this means that the maximum length of a string is also 255 characters.  There are many ways around this, absolutely none of which will work for code which needs to be server-side only.  If a client-sided GUI is not an option, then your only option is reduce the length of your string - try replacing <color> tags with \c tags instead.