I much prefer the look of OS X Yosemite over Mavericks and earlier versions, but unfortunately it's taking some apps a while to catch up, Google Drive is a prime example:

In some cases it might be possible to drag and drop a replacement icon into the info screen (via "Get Info") but this doesn't work for Google Drive as it refreshes it's icon for the root shared folder every time the app starts.

Fortunately it's quite easy to change the icons over permanently and someone has even gone to the trouble of creating new icon files for a number of popular apps and shared them on Google Drive.

OS X uses files with the extension icns for folder and app icons, you simply need to find the offending file within the app package and replace it.

Changing Google Drive icons

1. Go to your Applications folder and find the Google Drive app.
2. Right click and choose "Show Package Contents".
3. Browse to Contents/Resources/lib/python2.7/resources/images.
4. Replace the files "folder-mac.icns" and "sharedfolder-mac.icns" with the icns file from the link above or another icon of your choice.
5. Quit and restart Google Drive.

After a short delay, you should see the updated icon: