A user had this question recently “I just need to color the Subject line in an email as a one-off. Any ideas?”

While this is normally easy – just create a custom view using automatic formatting – there are fewer options for one-off changes. A flag set to be due ‘yesterday’ would color it red. If you want to use other colors you’ll need to create an autoformatting rule. Color categories and color flags are another option, although they won’t color the subject.