Yes, you’re correct, it has been debated at length before. It seems fish eyes are somewhat different to ours and may be able to see colours, especially ultra violet. It’s said we can’t see colour in the water and red line goes invisible first! This is 100% false, I have done lots of tests, and even though (to us) the line is no longer red, it’s clearly visible as a grey stripe, even Fluorcarbon that’s thought to be invisible, is easily seen as a grey stripe, it’s all marketing hype to sell you expensive line. All line when about 6-8m deep is grey to us, but you can see it.