Dekopon is a late ripening mandarin. In your climate because of high temperatures the mandarins don't get fully orange. But because you are in the northern part of the country where there is some cold temperatures at night like 6-12 C during winter some coloration happens.
Indonesia has different climate zones with mountains and plateaus. On those high plateaus they have warm temperate climates with temperatures dropping to 5C or even minus. Once I climbed to 3000 m altitude In Bali and I could see that every 1000 m there is different vegetation, and on top of the mountain quite cold.
Maybe you should wait more in Japan the are done in Dec-Mar for you maybe earlier one or two months , but don't expect your mandarins to get the same color like there.
Last year I saw an orchard in northern Laos in a mountainous area not far from you, with very healthy and ripe mandarins. Also very little diseases due to its remote location. Because of colder temperature they were very sweet, juicy and orange.
