Ultramarine is used to describe things that are very bright blue in colour.
...an ultramarine sky.
ultramarine in British English
(ˌʌltrəməˈriːn)
noun
1.
a blue pigment consisting of sodium and aluminium silicates and some sodium sulphide, obtained by powdering natural lapis lazuli or made synthetically: used in paints, printing ink, plastics, etc
2.
a vivid blue colour
adjective
3.
of the colour ultramarine
4.
from across the seas
Word origin
C17: from Medieval Latin ultramarinus, from ultrā beyond (see ultra-) + mare sea; so called because the lapis lazuli from which the pigment was made was importedfrom Asia
ultramarine in American English
(ˌʌltrəməˈrin)
adjective
1.
beyond the sea
2.
deep-blue
noun
3.
a blue pigment originally made by grinding lapis lazuli to a powder
4.
a blue pigment of similar chemical composition prepared from other substances