I thought of creating a filter to mark messages as unread, but it's probably easier if, after you read a message, you just mark it as unread by pressing the M key. That way, you can also see if the colleagues have read it, if they have the default setting on their setup.

My colleagues are not using Thunderbird but Outlook webmail and it is configured to not automatically mark messages as read, so they were annoyed seeing me mark every new messages as read since I set up Thunderbird x) I'm already using the M key as a temporary solution but I hope we can have a more convenient solution in the future. The feature to disable this behavior already exists, it just needs to be implemented on a "per-account basis".

Are your messages being marked as read before you read them? That has been caused in the past by apps like Bitdefender. As mentioned above, you could make a filter for just that account that marks as unread, either automatically when getting new mail, manually, or periodically.

They're being marked as read as soon as I open them, or rather as soon as I select them since I'm using preview mode (F8). It doesn't look like filters can enable me to mark what I'm opening as unread without marking everything as unread. I just want to automatically mark as unread what I am myself marking as read against my will.

If you uncheck 'Automatically mark messages as read' in Settings/General/Reading & Display, messages remain marked as unread even if you open (read) them. They can be marked as read by pressing M.

Even though this would apply to all accounts, marking as read with M is not a great inconvenience. Otherwise, create a separate profile for the one account with that setting.