Compare the colours with regular fishing supplies under a blue light as exists underwater. I suspect you will find the lack of fluorescent dyes results in the colours appearing brown.
You are talking about the colour changes on
ALL materials that are very deep below the surface. All colours change somewhat once you go beneath the surface. Available light and water clarity changes everything. Very few fly tying materials have fluorescent dyes. And, nobody really know why fish react to colours not normally seen in nature.
All you can do is experiment and see what works best for you.
I have used those hair extensions in various streamer patterns with good results. I just add some dyed polar bear hair around the plastic hair and add a few strands of Krystal flash or other commercially made bucktail hairs.
I've dyed polar bear hair for over 55 years and not once have I ever used fluorescent dyes. They all caught fish at various depths. The secret to good fly tying is to use a variety of materials in the same fly. Use the cheap materials for bulk, and the more expensive materials surrounding it for a bit of shape, colour and flash.