The U.S. is divided into 10 sections. Colorado happens to fall into what is affectionately called "zero-land". Hence the Ø in our callsigns. See the map for others.
Now, our number designation being said as an "O" all to often in speech today, makes it easy to use that habit in HAM radio. Don't get into that habit! Zero an "oh" will either alert people to your inexperience or your illegal use of the bands with a stolen call.
Make sure you use "Zero" for Ø. Look at the map and see where people are.