The Subject, in AAM context, is a general definition for role, individual user, visitor or default access to everybody. Other words the Subject is who you manage access to.
Each time you change subject on the Users/Roles Manager area, the current subject banner is adjusted accordingly. This gives you the visual confirmation for who are you managing access to.
The current subject can be highlighted with two colors. The red color means that you are managing access to the highest allowed user level or to the default access for everybody. That is why be careful as you can accidentally restrict access to your user. Additionally the entire AAM page will have the light red background.
Notice! Access to AAM UI is customizable and you can grand access to certain features for anybody who is registered on your website. For example you can give ability for your Editors to manage access to Posts & Pages. For more information about this feature please check How to manage access to AAM page for other users article.
The blue color indicates that you are managing access for role or user that has lower user level than you do.