A bright clean blue made with a phthalo blue red shade. The pale shades make a great sky and works well with an"Unbleached" titanium (#40-43) for clouds and lighting effects.
This is what I think of as cornflower blue. It is slightly toward the red in tone, actually I would say that it is a blue without a leaning to green. Because of this the lighter tones are good for a receading sky.
This is an Ultramarine blue with just a hint of phthalo blue. The first is a shade made with a Raw Umber to keep it from going to grey. This range is mixed a little closer then our other groupings and in the future I need to make a couple more lighter tones.
This is a close mixture of a "cobalt" blue. The second tone, #431, is as close to a cerelean as I have seen. If your skies tend to the greenish then this is the color for you.
This is a mixed group of intense deep blues. The first, #510, has a hint of green that works with the "complement" red shade of the blue to make a deeper tone without a hint of black. Both of the next two, #511 & #512, have an addition of a strong violet. The #511 is a rich blue and the #512 leans heavily towards the violet. The #513 does have a pinch of black but is a strong rich blue.
This is a red a blue as you can get before it turns violet. The tones are about as cool as one can get. A great contrast with the #430's and a nice companion with the #80's.
My set of blue greys. The trick to making these was to use some red in them so they don't turn green. They are definitely blue but can be pushed to look grey in the right circumstances.

A set of blues I developed for my Thunderstorm Greys. A dark blue not nearly as grey as the 540 above gives way to a strong deep blue that as it lightens starts to go brownish also.