Cyan is one of the primary printing ink colors in CMYK process printing. It's a primary color in subtractive color mixing and a secondary color in additive color mixing (the product of green and blue). Pure cyan light has a wavelength of approximately 490nm.

Subtractive (printed) cyan	Additive (computer screen) cyan

Cyan looks similar to azure (a.k.a. sky blue), however azure's hue is closer to blue. A cyan sky tends to look too computer generated.