Happy new year to you too!
Hmm... you know what... I don't think there is an option to do this using the UI! How odd... an oversight in the design perhaps.
The good news is you don't have to start from scratch. The bad news it you will have to do this by fiddling with the XML. Close down Tableau and open your workbook using a text editor (preferably something like Notepad++) and search for the text
under the relevant worksheet, you need to edit the attribute "line-color-auto" -- change it to true from false and save then close.
Reopen the workbook in Tableau and you should be back to automatic colors depending on what's on the Color shelf.
P.S. Please save a backup copy of your workbook before you edit the XML just in case!
Thank you ever so much for that reply. That is really useful, and I had no idea about the XML file behind. It's helped in a couple of other puzzles I'd had, so I am very grateful.