well, just imagine the colour from white-grey-black, it indicates the risks of doing SEO. Of course, the white one is welcomed by the search engine but relatively not so effective in a short period of time .while the black one such as Keywords Stuffing, Cloaking,Doorway Pages,Link Farming,Page Hijacking,Mirror Websites,Buying Expired Domains and Selling Links will take high risk and once detected , will be heavily penalized!