This is a long story I guess. Quick synopsis:
Over time I've come to consider color to be one of the least important variables in fishing presentation in general. There are times, places, and reasons where color (or at least visibility, but sometimes hue) can matter. But I believe color is way more important to humans than it is to fish. I have seen MANY examples of this phenomenon over time. This is an inherent (and strong) bias that lure manufacturers take advantage of. The only reason there are 200 different color patterns available in a popular trolling spoon for trout is bc people are attracted to them (makes them go "ooooooo" and buy them) and they'll even afix magic properties to favorite or novel color patterns that have nothing to do with water and fish. People fool themselves very easily. I have lots of anecdotes I could share that show this.
It's not that I don't think that red has been disproven as a trigger, it's that I don't think it's been proven in any rigorous way. Too many variables. Fisherman are NOT reliable sources. I’ve seen WAY to much blind bias amidst the myriad of variables that exist in nature. Red head lures may simply be more interesting to anglers than to fish. And when they do outshine a more drab color, it may have more to do with visibility than "red". Or confidence in the angler.
In my mind, red gills, red spots, "blood", red spots, excite anglers WAY more than fish. Bass evolved catching bluegills and crayfish. I’ve watched many successful kills and have never seen blood. Now shad can be different, but even then, a damaged shad is more apt to be missing scales than bleeding. Now throw some bluefish in there (called “choppers”) and you may have something there.
Again, that's not to say that there are times and places; I'm just talking in general, where people should be concentrating their time (and mysticisms). The apple may be, is likely, one of those times when color mattered. The fact that anadromous salmonids gain a red sensitive pigment upon coming shallow, that reds (and oranges) are important in the visual aspects of nuptial ritual, that aggressive territorial males get the brightest color (females the palest), lead me to be believe that color (reds esp) helps identify rival males. This is true in a fish in which nuptial rituals are very well studied, the stickleback. Males are heavily patterned and exhibit red while females exhibit pale silvery color (lack of color) and individual fish use these patterns for recognition.
See the second image.
A pale silver dummy female lowered into the tank can elicit courtship in male sticklebacks, whereas one with a red spot gets attacked. There are many fish well studied in which imm males sneak past a dominant male to spawn by mimicing the color of females -namely having no color. This has been shown for bluegills as well as many freshwater cichlids and saltwater reef species. Many gamefish, including trout and bass, also have more heavily patterned males and paler more silvery females.
So… It was my speculation that that male brown bit that size 5/0 apple bc it was screaming RED! It was early September (early pre-spawn) and that large male (for that stream) was aggressively spanning the length and breadth of the pool chasing other trout it encountered. It wasn’t just displacing competitors from feeding lies, it never held a feeding lie for more than a few moments to rest. It appeared to me that hormones were flowing. When that apple drifted into view I was amused to see it rise right up and take an aggressive bite at it. My guess is, although I‘m curious, it would have done so with an orange, or a chartreuse tennis ball (also common flotsam in this section of urban stream). So I’m not sure if RED is the only game in town, but since all salmonids including almost all browns have some red (at least in spotting) that red is potentially socially significant and a likely target for aggression.