“why [wiki]water [/wiki]has no colour?”
An object appear coloured or colourless depending on the colours of the spectrum absorbed, transmitted or reflected by it. We see an object in the colour we receive from it. Water allows all the colours to pass through them and therefore we see it as [wiki]transparent[/wiki].
(From another point of view “Water is one of the most essential substance for the existence of water. [wiki]Transparent [/wiki]and colourless appearance of water tels about its [wiki]purity[/wiki].”)