If the sky falls we shall catch larks proverbial saying, mid 15th century; used dismissively to indicate that something will be attainable only in the most unlikely circumstances.
out of a clear blue sky without warning; unexpectedly (with reference to a ‘blue’ (i.e. clear) sky, from which nothing unusual is expected). Compare out of the blue.
sky-blue pink a non-existent colour; recorded from the mid 20th century.
the sky is falling a warning of imminent disaster, especially one which is regarded as unduly alarmist; the phrase comes from the nursery story in which Chicken Little and other animals repeatedly warn the king that the sky is falling down.
the sky is the limit there is practically no limit (to something such as a price that can be charged or the opportunities afforded to someone).
See also blue-sky, mackerel sky, pie in the sky, winter never rots in the sky.