The cones in our eyes—the cells that detect color—only pick up certain wavelengths of light. We can see red all the way through violet, but other animals can see different portions of the electromagnetic spectrum. Plenty of birds have plumage with ultraviolet markings that we don’t see, but their compatriots do. That’s because birds have four cones instead of just three, and that bonus cone allows them to interpret UV light as visible. To puffins, it probably doesn’t look the way it looks to us under black light. We literally cannot imagine what color UV looks like, because our understanding of color comes from the section of the electromagnetic spectrum that we see. UV light probably has a ‘color’ that’s not related to any color we understand. But to birds, it’s just another feature of the world.