Q) How does a car advertised as having 'solid' paint differ from one with 'flat' or 'metallic' paint?
A) Solid or flat paint is the same thing. It's the cheapest option when buying a new car, but your colour choices will probably be limited.
Metallic paint contains tiny flecks of metal, which makes it sparkle under light. It usually costs extra (between 300 and 2000), but highlights the contours of a car's body better than a solid colour.
Cars with metallic paint are generally worth more on the second-hand market, too.