The colored coat is sometimes referred to as the base coat. This is a layer of paint that holds the color in which the panel is to be painted. This layer covers the primer and is usually finished with a layer of clear coat.
The primer coat is used between the material the panel is made from and the layers of paint that will follow. In some cases there can also be filler underneath to make sure the panel is perfectly flat and smooth before starting to paint.
The minimum paint thickness is an indication that there is no paint left to polish and should be avoided as much as possible. The downside is that this thickness is difficult to monitor or measure. Going passed this minimum thickness will result in strike-through of burn-through.
Paintwork can, in some cases, become oxidized. Making it turn matte and slightly fade color. If untreated, it can eventually cause the lacquer and/or paintwork to completely fail. It is most common on red cars, but it can happen with every color.