For years, I have been sitting on two ideas because I have two different breeds of occasionally-invisible heroes in my alien djinn romances series.
My twin Djinn princes, Devoron and Deverill rely on colors that the human eye cannot see. For instance, human eyes cannot see the pee trails that incontinent-by-design shrews leave in the woods. It was never necessary to mention this until I tell their stories, because as far as I remember in my God Princes of Tigron series, they were only noticed from the POV of another Djinn.
The other sometimes-invisible hero is Viz-Igerd, King of the Volnoth. His species is amphibious, and they have squid-like skin which can change color at will. I could have described their skin as chameleon-like, but squid also use their skin for sexually aggressive flashing in addition to camouflage. There are artists who use "camouflage art", and it is really interesting, but would not work in the wild without a very lengthy set-up. One's ambush hero would have to live like a spearing-type Mantis Shrimp, and his love life would be decidedly dub-con. (Dubious Consent.)
All the best,
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