When you create a new task label, any member of the team can then reuse that label for any task in that client account.
But what if you want to save a common set of labels for all your client accounts? To accomplish this, you could follow these steps:
Make a new task and name it My Labels.
Create the labels you want to use and apply them all to that task.
Save that task as a template.
Then you can add that task to any client you want, and it will import the labels to that client account.