Red Battery icon on Macbook Pro
I've already resolved the issue so this is an answer/explanation:
Go to Battery in System Settings. At the top of that page is Low Power Mode. To the right is a dropdown menu. If it is on 'Only On Battery' and the battery icon is red, it means that a) the battery isn't charging b) you've opted for Low Power Mode to save the battery and c) you have opted to Increase Contrast.
If you want to stay on Low Power mode when your Macbook Pro isn't charging, but don't want a red battery icon, go to Accessibility/Display. If the Increase Contrast option is on, turn it off. The battery icon will now be Yellow. Ta Da!!
MacBook Pro 13″, macOS 13.7