What does '...and noon a purple glow' mean? Why purple?
Yeats is altering both midnight and noon into eerie images.
Here Yeats gives evening a "glimmer" which can be spooky since it is supposed to be dark at midnight. He also has the sky full of wings of birds. That's not natural for the night. Birds are in flight during the daylight hours, not at night. That would be more like bats fleeing through the night.
Purple is an interesting choice in colors for the noon hour. I suppose pink or yellow would be too common. But purple is a little darker and unusual for that time of day. It was most likely chosen for the same reasons that he used "glimmer" when referring to night.
Very warm. Glowing with heat. I like the idea of the heather but as this imagined scene is summertime (heather blooms later usually) I'm not sure if he is being as concrete as that. "Purple" as a colour has connotatins of royalty. So he's suggesting the richness and comfort he would experience in this idyllic location. Midnight is all a glimmer here in the Western Isles at midnight on a good summers day (Midnight at Innisfree would be the same) and there are still many birds around at that time of day. It's not complete dark at eleven at present and in another month in high summer we will have barely three hours of partial darkness Linnets are small songbirds once trapped by young boys as alternatives to the modern budgie.