If you are on windows you can use Device Manager. Start with the colorimeter arduino unconnected. Open Device Manager. Then connect the arduino. After the arduino is connect in you should see a new device appear in the Device Manager. If you click on it you should be able to get the com port number.
On linux you can look in the /dev directory e.g. run "ls /dev/ttyACM*" the colorimeter should show up as /dev/ttyACM0, /dev/ttyACM1, ... etc.