I did notice some classes are highlighted with blue color in my project structure. What is the reason?
To expand on tyczj's answer, if version control is being used on the project then file names may be highlighted in different colours in the file explorer, editor tabs and active changes tab:
- White - No changes to the file since the last commit
- Blue - File exists in version control and has been modified since the last commit
- Green - File does not exist in version control and has been staged for the next commit
- Red - File does not exist in version control and has not been staged for the next commit
- Yellow - File has been moved from its previous location in version control, but its contents have not been modified
- Grey (only in Active Changes tab) - File has been deleted locally but not in version control
So in your example you have two modified files that already exist in version control.
As Zoe points out, this reflects the default colour scheme in Android Studio and IntelliJ. These colours may be modified via
Settings -> Editor -> Color scheme -> VCS