Kaleidoscope is a story written by Sparklebeard and illustrated by Iron Eater. It originally ran in Yes, And 3 at http://s2b2.livejournal.com/314348.html, and is mirrored at http://www.shousetsubangbang.com/mirror/kaleidoscope/.
Nicola takes her girlfriend Girril out for a nice dinner, and she makes it a memorable night by wearing a very special outfit under her presentable outer layer. It turns out that public sex looks very different when you're a human dating an alien.
From http://fairyninjas.wordpress.com/2015/07/21/circus-maximus-ssbb-yes-and-3/: "An intriguing snapshot of an inter-species relationship with one human, and one alien of a sort that’s really turned on just by colors."
The original inspiration for this story was a throwaway mention of an alien who was sexually aroused by visual stimuli, specifically bright colors, in an unfinished Torchwood fanfic I read in 2005. Even though this was just a one-sentence reference that wasn't explored further, I was intrigued by the idea and have had it tucked in the back of my mind for a decade now; when I mentioned it on a panel about alien genitals & sex at Wiscon 2015 it rocketed back to the forefront again. I've always been frustrated that the author didn't expand on this concept and eventually I realized that if I want more weird alien smut in the world, the best thing to do is to make my own. The original concept for this story involved Nicola and Girril first getting together and would have included a lot more cultural notes as the reader learned them through Nicola's POV. While that quickly proved to be an uninspired plot, it meant I brainstormed a lot of detail that didn't make it into the final story.
Cuttlefish were the major inspiration for Girril's coloration, although I gave the Ri a larger range of color and patterning options. They aren't otherwise directly related to any aquatic animal. The color response to sexual stimuli and other forms of excitement is an automatic reflex much of the time, but with concentration one can learn to control that reaction somewhat, similarly to how most people can learn greater control over their breathing with practice. There are also chemical methods to induce brighter coloration or deeper contrast; those might be used by romantic leads in movies or hawked on late night infomercials promising to bring that old radiance back to one's relationship.
When two (or more) Ri are in a sexual relationship, they rely more heavily on the feedback loop of seeing a partner's high-contrast arousal patterns and being aroused in turn than on colors and contrast in clothing. There are certainly lingerie stores that feature fashions reminiscent of an explosion in a Lisa Frank factory, but it's a bit of a niche market for people in Ri-only relationships; most of the customers are people in mixed-species relationships trying to find ways to entice a Ri partner. Ri are used to activating optical implants that filter visual stimuli down to a manageable level in mixed company, but as is briefly mentioned in the story, it's considered rude to force filtering in a one-on-one or small group setting as minor visual cues can be harder to make out with filters activated. So while a human in bright party clothes at a dance club with Ri patrons isn't being inappropriate at all, they'd raise some eyebrows wearing those same clothes to board game night at a Ri friend's house.
Ambient bright or high-contract colors aren't generally arousing on their own; if Girril sees a brightly-colored abstract painting in an art gallery, she's not going to have an arousal response, and the Ri district isn't entirely monochromatic. This can vary from person to person, though, and if an image is intense enough it might have sexual or sensual associations, although it is unlikely to be considered obscene on its own. For sexually inexperienced or younger Ri, something as intense as a Magic Eye image would probably be passed around like an old Playboy.