As either user types, their contribution is highlighted with their user color. Their current cursor position is also shown in this color.
You can view which color represents which user in the Share banner.
Note: Change marks are automatically shown when you participate in a shared document.
To hide the colored change marks, choose View > Hide Change Marks
To change your user color, choose Coda > Preferences… , click Sharing , then adjust the Editing Color slider until you find a hue you like.