As HDE 226868 noted, it is rather difficult to find data on the transmission / absorption spectrum of LN2.
However, we can make some guesses at what it should look like over large depths, based on comparing its structure with that of water.
Water strongly absorbs red and infrared wavelengths because it has lots of degrees of rotational and vibrational freedom, with molecular energy level gaps in the red to infrared range.
Molecular nitrogen, in comparison, has only one degree of vibrational freedom, and a much stronger, stiffer bond. Thus, while liquid nitrogen and liquid water are difficult to distinguish in the quantities in which liquid nitrogen is typically produced on Earth, both being colorless, we can expect nitrogen to remain effectively colorless, with minimal visible absorption, at much greater depths. The color of an ocean would thus be approximately the color of whatever is under it, and whatever is reflecting off of it.
There remains one other coloring effect, though: Rayleigh scattering, which is what makes the sky blue. A thick atmosphere of pure nitrogen would be just as blue as our thick atmosphere of mostly-nitrogen-and-some-oxygen, and you could expect the ocean to take on some of that color by reflection. On top of that, Rayleigh scattering also occurs in liquids, so extreme depths of liquid nitrogen would appear blue in transmission--thus, ocean on the limbs of the planet should be tinted lightly blue, and a deep ocean which reflects light back up from the floor will have the color of bright features of the floor shifted towards red, while the bulk of the ocean surface will be slightly blue in compensation.
The most likely color of that floor is white, due to precipitation of nitrogen ice under pressure. So, a world with a shallow nitrogen ocean should look mostly white, with blue tinges from the nitrogen atmosphere on the limbs, while a world with a deeper nitrogen ocean (possible on smaller worlds where pressure increases more slowly with depth) should shift more towards the unsaturated (whitish) blue due to liquid Rayleigh scattering, with orange-red patches where the seafloor is highly reflective.
Of course, if there is anything dissolved or suspended in that ocean, the color could change drastically. LN2 isn't a very good solvent, but the ultimate color of a world with an LN2 ocean could be determined by whatever life ends up developing in it, if any, and then it could be pretty much any color you want; such life would have to use some other intracellular biosolvent, but that just means the nitrogen ocean will be to it much like what the nitrogen atmosphere is to us--a medium that it moves through, and uses for temperature regulation, but which is otherwise biologically inert.