It seems a joke, but it isn't: sea is blue because water surface reflects the colour of the sky (blue, indeed).
Then, if you want to know why the sky is blue, look for that question (it's very hard to explain it in few words, or in a few formulae: it involves refraction and trasmission of light; optics and electrodynamics!)
The colour of the sea is greenish blue because of the reflection of the sky
a greenish blue color
The sea water is a greenish blue colour, (The greenish colour comes from the abundant plankton that thrive within the water.)
turquoise
teal is a four letter word for a greenish blue color ^_^
greenish blue
it is a greenish color.
Turquoise is a greenish-blue color that falls between blue and green on the color spectrum. It is often associated with tropical waters and gemstones.
Yellow and blue can be combined together to make the greenish color. Also, yellow and dark, green can create that "greenish color."
Cyan.
Amber.
greenish blue