Why are some Android Studio classes highlighted with different colors?

Because you are using version control and you have uncommitted changes for those files

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.

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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

Highlighted color is for Version Control.

If you create new file and do not commit it to GIT it's Highlighted with RED color

If you change in a file that already committed, it's highlighted with BLUE color.