I haven't heard about this phenomenon with Belarus and Serbia before. I know that in Russia, white and/or European identity has a very complex history, so their case lends itself to explanation a bit more easily. Obviously, Russians are white, but ever since Peter the Great European influence in the country has been anti-Russian, and the political relations between Russia and the rest of Europe have also been very troubled. Consequently, an anti-European camp has developed which goes as far as renounce European belonging itself. As a side note, during the Cold War, the CIA hilariously made it a policy to attempt to undermine Soviet reputation in Asia and Africa by reminding those people that "Russians are also white".