Aqua (#00FFFF) is a very light, highly saturated cyan hue. In CSS, aqua and cyan are the same colour: full green plus full blue light.
In RGB it is made of 0 red, 255 green and 255 blue. Its HSL hue angle is 180° with 100% saturation and 50% lightness, while its perceptual OKLCH lightness is 91% with a chroma of 0.155. For print, a starting CMYK mix is 100% cyan, 0% magenta, 0% yellow and 0% black.
As a text color on white, Aqua has a contrast ratio of 1.25:1, which fails WCAG AA even for large text. On black it reaches 16.75:1 and passes AA for normal text. If you use Aqua as a background, put black (or very dark) text on it. The APCA lightness contrast is Lc 92 for black text and Lc -14 for white text.
Its perceptual complement is #FFD3D5, close to millennial pink. Colors that look most similar are electric blue, aquamarine, turquoise. People with red-green color blindness see aqua quite differently (ΔE 15), so do not rely on it alone to signal meaning.