when I delete a folder which is showing in a light color, message states folder is undouble. I delete it but it re appears when I log back in. How can I permanently delete the folder?

Not sure what you mean by "undouble". Can you describe that in other words? Who is your email provider? What is the name of the folder? Is this an IMAP or POP3 account? Is this a folder you created, or was it automatically created when you set up the account? Depending on the provider and the type of account, some folders may not be deletable, though you might be able to Unsubscribe from them, making them not visible in Thunderbird (but they will still exist on the server and may continue to collect messages, and you won't see them in Thunderbird.) Please describe your situation more precisely and I can advise you on the next steps/