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The color may have intensified because of the temperature, sunlight or nutriton and might become lighter like you hoped. This article explains:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-turning-different-color.htm

Possibly a greenish, hibiscus sawfly larvae damage. Early on humid mornings, you can find their greenish larvae on the bottom of the leaves, chewing between veins. Because they are not caterpillars, they cannot be controlled using organic pesticides like Bt (Bacillus thuringiensis) but you could try spinosad (apply when bees are not active). For a bad infestation, you can also try hand squashing, insecticidal soaps or a systemic insecticide like Imidacloprid. They are most effective if you start applying in May-June per label directions. The damage is typically aesthetic only and would not normally kill the plant.

Not sure which rose mallow you were interested in. These articles should help:

https://www.gardeningknowhow.com/ornamental/water-plants/swamp-hibiscus/swamp-hibiscus-plant-info.htm

https://hgic.clemson.edu/factsheet/hibiscus/

https://aggie-horticulture.tamu.edu/wildseed/35/35.7.html

Looks like there are some insects that suck the leaf juices off so consider taking a leaf sample to your local plant nursery so they can see it up close, identify it and the suggest how to control them. Spider mites, whiteflies and aphids are common problems. Some of those can be as small as periods at the end of this sentence so the local folks at your nurseries can help identify them more.