Possibly "The Color of Distance" by Amy Thomson.
The book opens from the alien perspective with the frog-like
inhabitants of the rain forests of a world called Tendu. The elders
of one of their tribes find a bizarre new animal on the verge of death
and perform a miraculous feat of bioengineering in order to save it.
The creature's name is Juna, and she's part of a human survey mission
to the planet. Her scout ship crashed and her crew -- clad in
environment suits -- had been forced to try and hike back to base camp
through the dense Tendu forest. Their suits ruptured during the trip,
exposing them to an atmosphere to which their bodies were violently
allergic. Everyone else died, and Juna expected to join them.
She didn't, a fact that amazes and horrifies her when she awakes to
find herself transformed into something resembling the aliens. She's
able to breathe without life support, has a strange skin that reacts
to her moods, and razor-sharp talons. After choking back her fear, she
begins making contact with the intelligent aliens, hoping against hope
that she can still get to her mothership before it leaves.
Somewhat different from your description, but the alien race communicates by altering their skin colour:
As with many Earth frogs, they can change their skin colour. What
started as camouflage and simple colour signals has evolved into a
sophisticated symbolic language, presumably along the lines of
Egyptian or Mayan hieroglyphs.
~ Edited from SF Site and SF Mistressworks
There definitely is a lot of sexual content. I won't go into the details, but here's how the protagonist reacts during one such scenario:
Her skin felt warm, as though a lover had been stroking her. She
opened her eyes. Her skin was a brilliant, metallic gold. Was this the
color of sexual arousal? The aliens were the same shade of gold. Juna
fought back a wave of panic. Did this mean they wanted to have sex
Also, the protagonist does transform back into her human form. This (weird) conversation between Juna and Bruce, her new "human" lover, should illustrate that.
“What are you thinking?” he whispered.
Juna laughed. “I was just thinking how good it is to have nipples again.”
“I wouldn’t have minded you without them,” he said.
“But it bothered me,” she replied.
“I could have gotten used to it, Juna. I was willing to try.”
You can read the novel in its entirety here.