Why Are My Flagged Emails Yellow?

That is because the flag is set at its right edge. And that's the new feature. In the latest copies of Insider Outlook, when you set a flag on an email, the entire email turns yellow in the List view.

How do I change the color of flagged emails?

In any Mail or Contacts view, select a flagged item. On the Categorize menu, click the color category that you want to assign to the item. If you don't see the color category on the menu, click All Categories. Select the check box next to the color category that you want to assign to the item.

How do I make flagged emails red?

For example, Outlook has a built-in rule to color red all items flagged for follow-up that are past their due date. Therefore, you can right-click an item, select Flag for Follow-up, then type yesterday for the due date and click OK. The item will automatically turn red.