I have never seen a large enough body of liquid nitrogen to know what colour it has. A bit of research into spectroscopy would probably come up with some answers.

The blue colour of our oceans is only partly due to the "blueness" of a large body of water (or ice - look at photos of old glacier ice). Most of the blue is the same blue as the sky, reflected.

The trace contaminants - be they organic or non-organic would probably dominate the colour of an ocean on another planet.

Tholins (HCN complexes) are brown, for instance.

Longer-chain hydrocarbons are bluish-purple with interesting visible-light flourescence. Have a good look at the fuel stream next time you fill-up your gas tank (unless you are in California, or somewhere with equivalent vapour recovery systems that stops you looking at what you're doing)