Compressed files/folders become blue.
Encrypted files/folders are green.
If you don't use a file/folder for quite some time (somewhere around 30~50 days, or so), Windows will automatically compress it to save space for cache and etc.
Do not worry, nothing will be lost and you still would be able to use it just fine.
You can disable this by doing this: open folder -> click "folder options" -> click "view" -> check/uncheck the "Show encrypted or compressed NTFS files in color" box.