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They benefit from extra water during the summer, just enough to keep the soil slightly moist, and they are very heavy feeders.

Usually feeding them will bring the color back. This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm

Hard to say what the brown spots might be without an idea from a picture but be aware that with shorter amounts of daylight since the Summer Solstice and cooler temperatures, high light plants like hibiscus inside may react by yellowing their foliage in the fall/winter. However, make sure that you are not over-watering by testing the soil before watering: insert a finger into the soil and if it feels moist or wet/soggy then skip watering that day. This testing becomes more useful as the inside of homes becomes more dry in winter. There is no harm in letting the hibiscus continue flowering and opening however many flower buds it decides to open. A south facing location will provide more sunlight. The hibiscus may need supplemental lighting if the location of the pot is too dark.