Blue is a colour that I can't do without. One of the three primaries , it's essential for mixing other colours , and also often occurs in nature on it's own - especially if you live near the sea.
Blue pigments come in warm and cool temperatures , transparent and semi opaque , and fine or granulating. Once you become familiar with the properties of each blue you'll naturally begin to reach for the right one for the job.
The six blues which I currently have on my palette?

Cobalt - this is a gentle, pure, non staining pigment- i don't usually use it on it's own, but it's nice to mix with  or glaze.
Cerulean Blue - a beautiful granulating cool colour, a favorite for mixing soft greys with texture.
Pthalo Blue - this one is a prima donna! An intense colour , transparent and very staining, it will take over if given the chance.  But again, lovely for mixing greys. quite a cold blue (tends toward yellow rather than red).
Ultramarine Blue- my all time favourite , a granulating pigment - mixed with Burnt Sienna makes gorgeous darks.
French Ultramarine - this is a warmer , purpler shade of UB, a lovely delicate blue.The one pictured is QOR brand.
Indigo - I've only recently discovered Indigo , unlike the Egyptians who used it for dying fabric some years back. It's a very intense, staining, transparent blue , which I use to make beautiful darks .

Apart from the French UB, the other blues shown are winsor and newton. Each brand will differ slightly, but you'll soon find your favourites.
So, forget winter, and play around with some colour mixing....now, which one will make the most summery sky?