Do you have an invoice in your Approval Tab or Review Tab which is in red?
Why is the invoice line red?
This is because you have returned it from either the Signed Off tab to the Review Tab or from the Review tab to the Approval.
Once you approve it or sign it off again it will return to normal.
This feature is usually used when communicating back and forth between tabs.
When this happens, it is good to give your activity tab a quick look. That way you are aware of where the invoice has came back from, who sent it back and maybe the reason why if the last user has added a helpful note.
Then look down through the audit trail to find when it was returned.