I have three skull caps:
A Headsweats for 57F to 100F. It's very thin, keeps the sun off my head, soaks up sweat and is very cooling on hot days.
A medium weight winter cap for about 50-60F
A heavier Pearl Izumi skull cap for about 35-52F. It doesn't quite cover the bottoms of my ears, but for temperatures 45F or less, I add an inexpensive fleece "neck gaiter". It's great, I can pull it up over my nose on cold downhills, or scrunch it just around my neck if I'm getting too hot. It overhaps the cap near my ears, keeps cold drafts from going down my neck, too. I like the combo of cap and neck gaiter instead of a balaclava.
There's not a lot of temperature overlap between these. But in springtime, acclimated to the cold, I can go 5F colder on either of the thinner caps.