Watch dials that have aged and discolored due to exposure to light (usually black to brown) are called tropical.
Many dive watches, particularly the iconic Rolex Submariner, have black dials for legibility. Since these are often exposed to extreme, long-term sunlight, many have aged and faded over time. The materials used in the 1950s and 1960s tended to fade to a dark brown color and are known as “tropical” dials today.
Today, collectors prize watches with “patina”. Sports watch enthusiasts are particularly fond of “tropical” dials on models like the Rolex Submariner, Omega Speedmaster, and so on. This has led to some unscrupulous people artificially aging dials or hiding evidence of water damage while preserving discoloration. Therefore, collectors look for uniform aging of the dial, lume, hands and markers, bezel, and case.