There’s no shortage of good info about how and what a trout sees while feeding.  I know because I recently rabbit holed the subject and found myself deep in web searches and YouTube videos.  It all started while looking through my tying desk and pondering how to use the materials that had amassed over the years but were not yet tied to hooks. Being homebound, the creative juices were flowing and the urge to spin some new patterns emerged.  While looking through drawers of beads, feathers and synthetics the thought came to me, “What does a trout really see when deciding to eat (or not eat), say, a Rainbow Warrior vs. a Zebra?”  How does it really choose?  Knowing full well I was part of an enormous group of fly fishers wondering the same thing, I hit the Internet.  Articles and videos abound; some science based others experience based but most a combination of the two.  Many of them are incredibly interesting and seem to be pretty accurate based on my experience.  After a pot of coffee and a few hours of reading, I focused on repetition and narrowed it down to two good posts (linked below).
To save you some rabbit holing, here are some nuggets I found assist creativity on the vice while tying productive flies.  Disclaimer: due to time constraints during guiding season, some guides will tie up patterns that are relatively quick and require less materials yet still work well.  I wanted to explore new patterns and yet stick to the “less is more” philosophy.
Trout’s eyes are shaped to accommodate two focal lengths.  The small notch in the front edge of the pupil allows for close vision across the bridge of the nose.  The second focal point is the larger lateral part of the pupil that sees longer distances to the side.  The trout is able to process a dual field of vision, a forward focal point and the larger periphery.  Within the retinas are rods and cones similar to ours with a few differences that I’ll address later.  The rods sense a monochrome, black and white spectrum.  This is active in the evenings and nighttime with no color recognition and the color receptors are dormant.  During this time, accurate silhouettes are more important than colors although black with light colored accents for ribbing, wing cases or wings provide good contrast.
The other parts of the retina are the cones.  These are the receptors that distinguish colors.  The visible RGB spectrum of trout ranges from a peak of 440nm in the shorter wavelength Blue - Violet zone to 535 in the Green zone to 600nm in the longer wavelength Red zone.  Humans, like trout see 440nm in the Blue - Violet zone and 535 in the Green but 565nm in the Red.  This means trout can see longer wavelength “Redder” shades than we can.  This assumes an environment of clear water and good sunlight.  A quick note on the visible UV range of trout, which peaks at 355nm.  Studies reveal that trout seem to lose their UV ability after 2 years and that this receptor disappears. 
However, some biologists suspect it is only dormant and returns during spawning seasons.
Water Clarity and Available Light:
Trout visibility is obviously limited by water clarity and the quality of light entering the water.  Their extra sensitivity only helps if the full spectrum of sunlight is available and can reflect the colors we are presenting.  The shorter blue and UV wavelengths are easily dispersed, especially if impurities are in the water.  This means they are only visible at shorter distances.  The longer wavelengths of the red spectrum can be absorbed as heat and those deeper reds appear black at longer distances.  However, at shorter distances in direct sunlight, reds appear brighter to trout than what we see.  White reflects all visible wavelengths and is seen at longer ranges meaning flashes of tinsel and wire will be seen before the actual color of the fly.  White and flashy are productive when impurities cloud the water or you’re fishing deeper rigs.  Other colors are less important in cloudy water because impurities block the wavelengths needed to illuminate our flies.  However, dark flies providing good silhouettes are effective in deeper and cloudy water.  The caveat is they must be presented closer to the trout as they are not that visible further away.  Specific color discernment seems to only happen at close distances in clear water with good light.  This translates to contrasts of shades being more important in most lighting conditions than specific colors.  Therefore, mixing color schemes that highlight features of the fly and accent their shapes may increase their visibility to feeding trout more than considering specific colors themselves.  This concept is more pronounced in the Bluer spectrum, then the Red and lastly in the Greens.  In recent testing, I’ve had good success with a white body, chartreuse green ribbing, peacock hurl collar and a rainbow tungsten bead (see photo).
A Quick Word on Fluorescence:
When shorter UV waves are absorbed by the object’s surface and converted to longer wavelengths the effect adds intensity (fluorescence) to the already visible colors.  In off color water and sunny conditions fluorescent red, orange, and yellow are highly visible.  In clearer and deeper water under sunny skies fluorescent and chartreuse greens become highly visible, as well.  In deeper cloudy water and overcast skies the UV rays can be filtered by impurities and are less likely to illuminate a color’s fluorescence thus negating the intensity.  However, in conditions like faster moving, shallow water where light waves are refracted in many ways and many angles, fluorescence can work quite well.
Imitating the Real Thing:  So why do trout eat things that have color schemes different from real bugs?  I have to rely on the explanation given by Jason Randall (linked below).  His concept of the “search image” aligns well with my personal experience with fly patterns and success.  He claims a trout is looking for four criteria:  Size, Profile, Color and Animation.  If all four are present it’s likely a feeding trout will eat it, “providing it imitates a food they’re currently eating.”  He states this list of criteria is an inclusive list not an exclusive list.  Meaning it’s not a negative if the presence of something not on the list is seen , i.e. a hook.  So if a trout refuses a fly, it may be the absence of one of the four criteria rather than the presence of some other thing.  We’ve seen many a selective trout nose up to a dry and quickly refuse it.  At this close range color could definitely come into play along with the other three criteria.  Maybe our dry was missing something the trout wanted to see rather than having something it didn’t like.  Makes me wonder how many refusals occur while nymphing.
In Summary:
Pondering this information while sitting at the vice, I’m encouraged to experiment with color schemes that emphasize contrasts more than specific colors in and of themselves.  I will say that lately patterns focusing on this concept have produced quite well.  Materials and colors that have been shoved to the back of the drawers are now back in the limelight.  As of now spring is on the way and runoff is pending.   I look forward to experimenting with new patterns and encourage you to do the same.  Try new things and as you peruse the fly bins at the local fly shop, give a nod to flies with unorthodox schemes.  Who knows, it may be the one that produces well when presented to selective fish at popular spots who’ve seen thousands of the “same ‘ol thing”.

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