Purple (P) is like Orange (O) in one way - it is the only other color that has no rhyming words in modern English usage (don't start with "hirple"). Otherwise, P is utterly different. P is not a color in the visible spectrum of light. Huh?!
Here is the visible spectrum:
You will note that the colors that are visible to most adult humans (children can see into the ultraviolet, as can those who have the lenses of their eyes removed in cataract surgery) range from the Red to Violet. We use the acronym ROYGBIV to remember them - Red, Orange, Yellow, Green, Blue, Indigo, Violet. No purple. We consider Orange a distinct color, but in reality it is a merging of Red and Yellow just as Yellow is a merging of Red and Green. In some light physics, the three primary colors of light - Red, Green, and Blue - are used exclusively, since all other colors may be considered derivative. [Yellow has a specific range of frequencies/wavelengths, but when a yellow wavelength strikes the cones of the retina, it will be accepted by both Green and Red cones - we have no Yellow cones, just Red, Green, and Blue. Each of these adjacent cones will pass up the intensity of their respective R and G and the brain will say "Yellow". Smart brain!]
So, Purple is not shown on the spectrum. That is because it is the join of R and B light, which are not adjacent on the spectrum. R and B actually cancel each other out, in a sense. In my days of theatre lighting, we could allow someone to disappear from a scene by overlapping R and B spotlights on the stage. The actor could walk from the R into the point of overlap with B and not be readily seen in the darkness (Purple). Using light to make darkness, weird!
Now to P and water. Because P is R + B [HTML defines Purple as R -128, G - 0, B - 128] and R is the first wavelength lost to depth of water and B lasts quite deep, P will appear as Blue in deeper water (without factoring in UV). However, in shallow water or close to the trout, P will be fully P. On cloudy days, or milky water, the P will stand out well as a very dark object against the ambient backdrop.
When we say Purple we are usually considering the Snipe and Purple. Try as I might, I couldn't find any two ties of the Snipe and Purple that used the same body color. Some used a dark, almost black, Purple, others a Royal Blue - the range was endless. For convenience sake, let us assume that the body is truly dark P. Why would dark Purple or dark Blue at depth, attract fish? Because the iridescence bodies of some species of blow-flies, beetles, nymphs, etc. are Purple. The Blue Bottle fly is found worldwide -
[Notice how hairy some bugs are? Think peacock herl.]Now, I am not saying that the trout mistake a Snipe and Purple for a Blue Bottle Fly, but the coloration of both is similar and the Blue Bottle is common everywhere. Obviously, the insect is chubbier, but if the trout only sees a flash of color, that may be enough.
From a UV perspective, the remarks made about the Partridge and Peacock apply here as well. The snipe will dependably reflect a lot of UV (assuming it is a natural product, not dyed), the dark purple will, generally (dye is a consideration), reflect little, - but depth may also play a factor because the UV won't have any cancellation by Red at depth, so the result may be the body appearing lighter in the UV in deep water.
