Waxes
These are top coats and shouldnt be abrasive in any way. Avoid the polish/wax combo products and the coloured products. They can be liquids or solid pastes, but all should contain Carnubra wax if they are of any quality. This is the hardest natural wax and its use will mean the product will last for a month or two if not more, not a couple of weeks. The good old fashioned paste waxes are great on dark paint, they are slightly warming to the finish, adding great depth. If your paint is now prepared correctly they should glide on and off quite easily. The liquid waxes are often hybrids, with both wax and a synthetic in them. They are fine as well, but avoid anything that calls itself a polish. Liquid waxes are usually made for ease of use.
Many things claim to be waxes when they arent. If its perfectly clear its not a wax, if it drys powdery and white its not a wax. Again few things sold in Halfords are really waxes. They should be what you expect, yellowy liquids or pastes... that look like waxes!
Prices for waxes varies morew than for any other stage. A cheap paste wax can be a few pounds, a custom made high end wax can be £3000. Is it worth it? No, not for its performance anyway.
Examples of good waxes are Collinite 476S, which is very durable and very hard, the Auto finesse range, the Dodo juice range and if you are feeling rich, Swissvax whos cheapest wax is about £65 and most expensive non custom product, £800!
All will look great on dark colours and if pthe paint is prepped properly should give a stunning gloss.
Synthetic Sealants
This is a really complicated category, but basically a sealant is any top coat that isn't made from natural waxes. They can vary from wax like pastes to thin oily liquids and milk like fluids. Like waxes they shouldnt dust up when dry, they should only go hazy and should not have any cleaning or abrasive ingredients.
There are some really funky sealants out there, some are simply synthetic waxes like FK1000P, which is great for silver, some are spray on milk like liquids like Werkstat Jett. Others are oily liquids that deposit hydrophobic nano crosslinking bridges behind, that repel water intensely. There are even ceramic based products that are harder than your paint and will genuinely last years. All can be used on any paint colour, but synthetic sealants are especially good on light colours and light metallics like silver, because they enhance reflections and depth but dont warm them, like waxes do.
Conventional sealants are things like FK1000P, Auto Finesse tough coat, Optimum opti-seal, Werkstat Jett Acrylic and (confusingly) Meguiars Ultimate wax. They are spray on or spread on and just need wiping off like any wax would. Some people are dubious of the spray on types, thinking they cant possibly work, but they do, they really do.
The Nanos are a different type of product. The bond to the paint and are really hard to remove, in fact they are resistant to acids and even petrol and many solvents. Wolf Chemical Nano Body wrap is an example. Its very durable, giving many months of service. There is now a body wrap hard body, which is harder and can last 2 years. This needs applying with a cotton wool pad and removing with a microfibre. The effect is stunning.
There is also Permanon. This is very very easy to apply, just spray on and walk away. Its similar if less durable.
Beyond that we have the cermanic hard coatings. These are top of the line, state of the art sealants that are harder than your paint. Examples are Concours Car Care Ceramishield, Optimum Opti-coat, G-Techniq C1 and CarPro C-Quartz. These leave behind a silicon (not silicone) coating that resists swirl marks, scratches and chemical attack. They can only be removed with a compound and a polishing machine, they are that tough.
They arent THAT expensive, ranging from £30 to 50, but thats for one cars worth. Remember your car will be protected from scratches and wont need waxing for 2 years minimum. These are not the rip-off dealer applied coatings that are just average sealants, they are the real deal.