This could very easily be an effect of limited resources coupled with the Denizens not needing to see only in visible light. Technically, wavelengths in near infrared and in ultraviolet can also be seen with eyes and other organs alongside visible light. (For example, reptiles and birds can see as far into the ultraviolet as much as an average human can see into red compared to a dog.) In fact, some animals, including a select few mammals such as platypi, can fluoresce in dark light (they do make dark light flashlights out there). Imagine Leonard glowing pink!
But the Denizens are neither mammal, bird, nor reptile. They probably can see into electromagnetic wavelengths no animal can. As UV would cause Leonard to fluoresce in visible light, the output of a Denizen reactor would cause a lightbulb to "shine" these other wavelengths which would cause things to "fluoresce" in ways that would be utterly doubleplusWEIRD to our eyes, as Scarlet so ably demonstrates.
Of course, the question arises of how a perfectly ordinary light bulb can possible "fluoresce" in way it isn't meant to without being modified in some way first. I'd say it is
being modified, by some intrinsic add-on to the electrical current that affects the light bulb while the Denizen power is being applied to it. This likely is hardwired into the reactor's power supply output so it can't be simply bypassed. But turn the current off, and the light bulb does revert to being ordinary again. The Think Tank can filter out or work with the add-in with their own high-end, lab-quality equipment downstairs, but bog standard household stuff wouldn't stand a chance since the add-in is likely designed specifically to affect it. Plus, the "fluoresce" wouldn't be noticeable in standard Denizen tunnel lighting any more than Leonard glows pink in sunlight.
Likely, the Denizens did this because they need that add-on in order to work with their equipment, so asking them to remove it would be like demanding a human filter their eyesight so they can only see in black and white (not just grayscale, but only
in black and white).
And who knows? Maybe I'm just whistling in the dark.