I have just encountered an issue where in Mac OS X 10.10 all small folder icons changed from blue to the white paper icon. Large icons were fine, just the smaller ones. I found this link that finally helped me as a restart did not.

Basically, perform these steps:

sudo find /private/var/folders/ -name com.apple.dock.iconcache -exec rm {} \;
sudo find /private/var/folders/ -name com.apple.iconservices -exec rm -rf {} \;
sudo rm -rf /Library/Caches/com.apple.iconservices.store

Then relaunch Finder.