Is there any opinions on certain brands of wax working better with certain colors? For example, is there a specific brand/type to use on gray cars, or is there no difference across brands or types?

 

I've owned many cars and many colors. Super black bright white and everything in between. I'd suggest meguires paste wax you can't go wrong with that.

 

In my idea. for light colored paint it would be better to use a paint sealant. And for dark colored paint pure carnuaba wax would deepen the color and shine.

 

The color of your car will not decide what wax or sealant you should be using. Wax and sealants are not used to protect the "color" or base coat of your car but the clear coat. To make this understandable as well is.. NO wax or Sealants will give a real "shine" to your car.
The most a wax or a sealant will do is fill in the swirl marks on your car to give you the illusion of a smooth clear coat. After the wax or sealants wears of from weathering, the swirl marks will reappear, which give your car the non-shiny dull look. Buff your car with an orbital buffer with the correct technique and then use either a good sealant and wax in that order to give your car a long lasting shine.

 

Well not all wax and sealant has filling capabilities. That is why you need to polish out the paint defect before applying any protection.