Hi Ellen!
I'm Jen, an independent advisor and a Microsoft user like you. I'd be happy to help you out with this issue.
Try to create a copy of the entire sheet. On the new sheet, try to change the colors of the trendlines and save your work then check if the new changes were saved. You may check out this similar concern in the community forum.
https://answers.microsoft.com/en-us/msoffice/fo...
I hope this information helps. If you have any questions, please let me know and I'd be glad to assist you further.
Thank you and Best Regards!
Jen :)