Grey hat SEO is a combination of white hat and black hat SEO, where white hat SEO technique is used in access so it is kind of becomes a black hat SEO
At the risk of exposing Grey hat SEO here is a list of some of the things that are technically legal, but fall outside the scope of white hat SEO:

1. SEO Squatting

Buy up expired domains which are relevant to your keywords and then add some pages of content with a few well-placed backlinks to your site. Make sure that the new content is good quality and not too different from what the website hosted before. Put some adwords on the new content and then do some satellite SEO on it to keep up its trust rank.

2. Create Social media accounts for your pets.

Really this works. Get some cute stories and pictures, then one day your pet suddenly develops an interest in power tools, or mobility aids, or whatever website you are trying to promote. This will then create social search links back to your website. Not against Google’s rules to do this, but could contravene the social media that it is hosted on. Then again, who's going to sue a dog?

3. Redesign your website at regular intervals

Even if the content is more or less the same, because the code and text changes, google thinks that this is fresh content.

4. Add a comments box to your site pages and invite anyone to comment.

Sure it could get you 9 million spammers to put their links on there, but if you only approve the less spammy ones and then replace their links with a rel=nofollow tag, this means that you get no backlinks bleeding your page rank, but what you do get is lots of fresh content with hopefully relevant text for no effort on your part.

Google always support white hat SEO is preferred as it is more reliable and healthy for your website