A workaround for changing the Insync google drive folder icon I use is replacing the following image files with the ones you prefer, e.g. by deleting and symlinking each to an appropriate one in your current icon theme (folder-google-drive, folder-cloud…)
/usr/share/icons/hicolor/16x16/places/insync-folder.png
/usr/share/icons/hicolor/22x22/places/insync-folder.png
/usr/share/icons/hicolor/24x24/places/insync-folder.png
/usr/share/icons/hicolor/32x32/places/insync-folder.png
/usr/share/icons/hicolor/36x36/places/insync-folder.png
/usr/share/icons/hicolor/48x48/places/insync-folder.png
/usr/share/icons/hicolor/64x64/places/insync-folder.png
/usr/share/icons/hicolor/72x72/places/insync-folder.png
/usr/share/icons/hicolor/96x96/places/insync-folder.png
/usr/share/icons/hicolor/128x128/places/insync-folder.png
/usr/share/icons/hicolor/192x192/places/insync-folder.png
/usr/share/icons/hicolor/256x256/places/insync-folder.png
As for the emblems:
It seems Insync uses some common emlems like these:
emblem-people
emblem-ubuntuone-synchronized
emblem-ubuntuone-unsynchronized
emblem-ubuntuone-updating
And some special emblems like these:
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-error.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-error-shared.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-synced.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-synced-callbacks.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-synced-callbacks-active.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-synced-shared.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-syncing.png
/usr/share/nemo/icons/hicolor/64x64/emblems/emblem-insync-syncing-shared.png
I’ve also changed them slightly (making smaller and adding some transparency)