Tropical is a generic term to describe a wide range of dial changes from what could be termed damaged through road tar up to a pristine dials that have even color. It is almost like diamond grading as the quality goes up so does the price. I use the term color change when describing a pristine dial that has color ranging from dark chocolate to caramel. Tropical dials for me describe the more exotic range in more generic terms. It's like crackled dial vs crazed. The crackle dial sometimes called spider is a dial that has cracks all over the dial. I don't like the term spider dial because it is pitched as more expensive, as if rare, to charge more money. When it comes to crazed dial they are like tropical at the lower end you have damaged dial with lacquer peeling of up to dial who's crazing need to be viewed with a loop that is the high end.
Just my personal observations.
Bill