Yeats is altering both midnight and noon into eerie images.
"There midnight's all a glimmer, and noon a purple glow,
And evening full of the linnet's wings."
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.