An architect or builder can edit the label legend for all grids in your account.
First, ensure you are on the grid library page. If you click the label action button (within the grid library action button bar - top right of page) you will see any pre-assigned labels. If doing so for the first time, the dropdown will say 'No labels assigned'.
To edit the grid library label legend, make sure you are in edit mode. The label dropdown will now reveal eight coloured labels. Click inline to edit the text of any of the coloured labels you seek to use. Note: users will only see labels in the grid library legend that have been assigned a title.