To be fair, a little more description than "yellow sheet" would have been helpful.
Yeah, fair enough. I think those who have dived using this kind of lights and filters likely knew right away to what that referred, but I can see how it could be obscure to someone who hasn't.
With the blue lights, stuff looks mostly blue, as you would expect when you point a blue light at stuff. The blue light is an excitation frequency that's absorbed and that stimulates light output at different frequencies (that's fluorescence) by various materials such as proteins in organisms. Things that fluoresce at that excitation wavelength are fluorescing, but that's difficult to see through all the blue bouncing back to you from everything that
isn't absorbing it and emitting a different wavelength. Using a filter to remove light of the excitation wavelength (all that blue) but not the fluorescence wavelengths makes it much easier to see the fluorescence. This effect can be achieved with an excitation filter over a sufficiently bright light (that emits the necessary wavelength) combined with a barrier filter over your eyes.
There are actually now UV dive lights available that excite fluorescence at colors we can see but the light from which isn't directly perceptible to humans. Those don't need the yellow filters (at least for humans - I don't know about various camera sensors) because the non-fluorescing stuff doesn't appear lit up. There's still a lot of light bouncing back to you, it's just that because you don't see UV you can't see that light. This requires a UV emitter and can't be achieved with filters over a regular light.
Different proteins fluoresce in response to different wavelengths, so it's not necessarily the case that UV wavelengths invisible to humans are better than blue excitation lights that require barrier filters. I'm not an expert and don't really know the limitations of each in terms of sport diving. Since I'm not doing research, and because I've lost a couple of the yellow mask filters, I'm especially intrigued by the idea of just being able to wave a light at stuff and see it fluoresce. I do notice that NIGHTSEA has lights that have white output in addition to blue so you can do the fluorescence thing and still be able to see other stuff and potentially carry only one main light. That's pretty cool, too.