Gray Hat SEO is combination of white hat and grey hat seo.

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Greyish Hat SEO is the exercise of using officially lawful ways to increase your site positions, but which are legally suspicious, and could one day become dark hat.

Gray hat seo is the ugly guy.
Gray refer to methods that are not totally disapproved to use by SE, but still not considered white.

Grey Hat Search Engine Optimization is the method of making use of practically lawful approaches to enhance your website positions, however which are morally suspicious, and also can someday end up being black hat.

Both black hat and white hat seo is called Grey hat seo.