It turns out that this is not a settled question, even in the real world. Local conditions can affect things enough that opinions will vary by geography, never mind personal perceptions. It also turns out that the Behr Android phone app and I have vastly different perceptions of what that sky blue colour is…
Long story short, I rashly tried once again to match a colour photographed under natural light with the app and did not get satisfactory results. My phone shows a nice sky blue and the paint is downright purple. Even with some added white it is never going to do except for perhaps as a participant in a spectacular western desert sunset.
I ended up finding an online discussion of sky paint colours and, for the record, went with somebody else’s light sky blue: Behr Serene Sky 540C-2. The failed match was Periwinkle P540-4. Interestingly, one of the reported sky colours in the found discussion was a periwinkle although not this one. No photo of the result as far as I could tell so who knows. I will stick to the pale white-ish blue suggesting a hazy summer day.
Here is the colour on first application with the failed contestant represented on the stir stick. You can see the some of the problem because even the sky blue looks purplish which is not representative of the in-person view at all. More photographic experience and fooling with lighting is indicated.