White Hat SEO is effective only for SERPs where there are no Grey Hat or Black Hat SEOs, Black Hat SEO is for high-risk, high-yield opportunities, and in most cases, the term SEO means either Black Hat SEO or Gray Hat SEO. The rest is just good web mastering.
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!
Gray hat SEO is SEO techniques that take more risks than white hat SEO techniques but aren't likely to get your site banned from search engines. They are questionable SEO techniques but not in the same category as black hat SEO techniques.