What products are you interested in? Wax or nano sealant? I'm a wax fan so I'll start with those.
WaxMy all time favourite waxes are DoDo Juice Pro Edition waxes, I simply don't think you can beat these for the balance of - superb finish, a deep glossy shine, good durability and longevity, ease of application and price. For a blue or dark car the two to look at are:
DoDo Juice Purple Haze Pro (paste wax)
https://www.amazon.co.uk/Dodo-Juice-DJPPP30-Carnauba-Purple/dp/B003JJ4KVO/ref=pd_cp_263_3?ie=UTF8&refRID=FABR4MGMES5F8YMX7CKVand DoDo Juice Blue Velvet Pro (hard wax)
https://www.amazon.co.uk/Dodo-Juice-DJBPP30-Carnauba-Velvet/dp/B003JJ3H5E/ref=sr_1_2?ie=UTF8&qid=1461365563&sr=8-2&keywords=dodo+juice+blue+velvetThese links are for the smaller 30ml sample pots which will give you a few full car coats and is a cheaper way to try it out than buying a full pot straight off. Both are great but I think the Purple Haze would especially suit a Carbon Grey car. Blue Velvet good too but probably the better choice for a black car IMO. There are two versions of each. The standard non pro edition which are good but the Pro Editions are better, longer lasting and give a beautiful deep wet gloss to paintwork. Both should last about 3 months on a car.
I also really like DoDo Juice Supernatural Hybrid - a wax/sealant hybrid. I used it a lot but find it a touch too sterile or lacking in depth for a black car but it could work very well on grey. Here's the full size but you can get a 30ml panel pot too.
http://www.cleanyourcar.co.uk/wax/dodo-juice/dodo-juice-supernatural-hybrid-paste-sealant/prod_887.htmlOther than that I highly rate Collinite waxes - 915 Marque de Elegance for dark cars but all their waxes are good including 845 and 476s. They are pretty durable too.
http://www.cleanyourcar.co.uk/index.php?searchStr=collinite&_a=viewCat&Submit=For a really quick and very wet look liquid wax, Chemical Guys Butter Wet Wax is also a winner, but it has quite a short lifespan, but great for a regular top up through summer and a dream to apply
http://www.cleanyourcar.co.uk/index.php?_a=viewProd&productId=141I myself am going through a flirtation with Swissvax waxes at the moment. These are pricy and not very long lasting but I thought it was time to give them a go. I'm liking them a lot.
http://www.theultimatefinish.co.uk/car-care/natural-carnauba-waxes.aspx?manufacturerid=29&gclid=COjW8N2ro8wCFc0y0wodkHEB4gAll of the above will be fine applied to very clean paint, by all will last longer when applied on top of a base sealant such as my favourite Poorboys Ex-P which is the perfect under sealant for a wax.
http://www.cleanyourcar.co.uk/index.php?_a=viewProd&productId=26So if using wax - whilst the base sealant is not absolutely necessary, if you were to use it the application would be as follows:
Base sealant - e.g. Ex-P
Optional Glaze
Last Stage Protection (LSP) - your chosen wax
GlazeGreat for dark cars - Poorbiys Black Hole.
http://www.cleanyourcar.co.uk/glazes/poorboys-black-hole/prod_494.htmlSealants Then there's sealants such as Gtechniq C2v3 or Sonax Brilliant Shine Detailer, Gyeon Can Coat, CarPro Reload etc, some of which have a cult following amongst nano/polymer sealant fans. Sealants are non colour specific like some waxes.
http://www.cleanyourcar.co.uk/index.php?_a=viewProd&productId=1040http://www.cleanyourcar.co.uk/detailing-sprays/sonax-xtreme-brilliant-shine-detailer-750ml/prod_1178.htmlhttp://www.cleanyourcar.co.uk/sealants/gyeon-q2-can-coat/prod_1638.htmlhttp://www.cleanyourcar.co.uk/sealants/carpro-reload-spray-sealant/prod_782.htmlObviously there's many more of either (wax and sealant) out there. Take your pick.