JF: I'm not going to argue with Gary Lipon about whether Shade's iridule and Nabokov's whatever was more likely to be a sundog (my suspicion, not conclusion) or cloud iridescence.  I agree with his point that sundogs don't look that much like rainbows, but neither do iridescent clouds, in my experience--I've certainly never seen a "bow" shape.  Either way, it may be amusing that Kinbote is right for once.  However, I will say that cloud iridescence isn't rare at all--one can look for it whenever the sun is shining through or next to thin clouds--and if he hasn't observed it yet, he may enjoy doing so.
 
I'm afraid I can't see Shade as using a metonymy to define a scientific phenomenon that readers are presumably unfamiliar with.
 
Jansy Mello mentions her enviable experiences in Brasilia.  I must say I don't think any place on Earth has the sun at any different angles at sunset to the horizon than any other, but maybe Brasilia's climate favors the kind of weather when these phenomena appear, and unquestionably some people observe them better than others.  Also, rainwater can certainly reflect a rainbow (it must have been spectacular!) because it can reflect anything, but clouds can't reflect an image of anything.
 
I can see that Nabokov might have made Shade inattentive in referring to a rainbow (though the mention of a thunderstorm suggests a real rainbow), but I wonder what Nabokov and Vera said in their letters.  I'm sure real rainbows are indeed common in Telluride during the summer.
 
And I don't see why the peaceful scenery, not the thunderstorm, is a deceit.
 
Jerry Friedman
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GL: I greatly appreciate the references, but I disagree with the conclusion. I think the phenomenon is simply cloud iridescence. My previous thought was it was a hoax that pointed to how much the reader is willing to believe the words of a writer or one of his character. I was pretty certain it wasn't a sundog, they're too different. But shimmering pastels sounds rather rare and wonderful to me, and would be something likely to be remembered. 
The definition that Shade gives is to be taken metronymic-ally: serving merely to associate iridule and rainbow, the container and the contained.
I'm voting for cloud iridescence.
 
JM: Thanks to Gary Lipon because he returned to the fantastic  "iridule" image in PF: "The iridule — when, beautiful and strange, / In a bright sky above a mountain range/ One opal cloudlet in an oval form/ Reflects the rainbow of a thunderstorm/   Which in a distant valley has been staged ..."
I've seen rainbows in Brasilia that look like wide open three-dimensional arch-ways (not flat ribbons), something perhaps caused by our location in relation to the setting sun. Parhelions lie "parhelially," ie, usually at the opposite side of a rainbow and, mostly, against a "bright sky" as Shade's "iridule". 
 
Incessant rain over wide watery surfaces, in the setting sun, extend the rainbow-colors all over the landscape (once, driving over a bridge, I felt I was inside an iridescent crystal hemisphere) and, in this particular case, they differ from "cloud iridescence" for there's no cloud standing in isolation, nor the refraction takes on the shape of a bow*.  However (who knows?) this peculiarity may be tied with the formation of Shade's cloudy iridule, since a rainbow can also be reflected onto a glistening surface right under one's feet. I'd have believed in the "physical reality" of Shade's description were it not for his powerful metaphor (the turmoils of a distant thunderstorm can be reflected as a deceitful peaceful scenery, for example...)
 
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*- perhaps Nabokov is indicating the unattentive insistence when applying the word "bow" automatically when, what the eye is then registering, is  only an "iridescence". 

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