Rename Color Categories of Shared mailbox

Hi,

I have created a Shared mailbox and would like to grant permissions for specific users to use the Color Categories. 

So far I've granted my test user (sulava.admin) Full Access to the mailbox with PS:

Add-MailboxPermission -Identity mysharedbox -AccessRights FullAccess -User sulava.admin

I've also tried adding sulava.admin user as Owner for Calendar folder using Outlook (based on these instructions).

User is able to add/remove/use Color Categories, but renaming category is grayed out. What additional permissions need to be granted for the renaming to be possible? Tenant has been upgraded to v15.

Thanks!