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Grey hat SEO techniques can be defined as the combination of black hat SEO techniques and white hat SEO techniques but it is not that much harmful that a black hat technique can do for a website but still we cannot say that it is ethical way of promotion of a website. So webmasters don't go for gray hat techniques.
Gray hat SEO of a combination of white and black hat.
It is batter then black hat and bad then white hat.
Since the color gray is between black and white, logically Grey Hat SEO sounds like a label for the middle ground. But it’s not. Because White is pure white and grey is a shade of black, we have confusion. Some say Grey Hat is NOT White Hat and is just a shade of Black Hat. So let’s step away from the color wheel and define Grey Hat as the practice of tactics/techniques which remain ill-defined by all that published material coming out of Google, and for which reasonable people could disagree on how the tactics support or contrast with the “spirit” of Google’s published guidelines.
Gray Hat SEO is about risk versus reward. Some Gray Hat SEO services tend to use more dubious devices and take bigger risks. 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 (although a search engine penalty could result).