I'll strictly refer to OEM Nissan coolants here.
They are ethylene glycol salt based, non-silicate


To answer the big questions:

• You can use either green or blue coolant in your vehicle. Blue has a longer life.
• Yes, you can mix green and blue coolants. The color mixture will have the life of the green coolant.

Further explanation below:

999MP-AF000P is the part # for the green coolant.
Usually NOT diluted. Mix with distilled water in a 50/50 ratio before use.

999MP-L25500P is the part # for the blue coolant found in newer Nissan vehicles.
Usually prediluted. Just use in the car straight out of the jug.

999MP-L255QTP is also blue.
Confusingly called a concentrate but the jug says you should not mix with water. It also states "dealer use only".
*It would be best to avoid this one.

And here is that literature on coolant/antifreeze longevity:
Nissan TSB # NTB09-04a or Infiniti TSB # ITB09-029a

In summary:
1. Blue coolant
Long life stuff, it comes in newer models from the factory.
Nissan sources mention that from the factory, it will last 10 years/135,000 miles. Elsewhere it also says that from the factory it is good for 7 years/105,000 miles.
However, if you flushed and filled with this coolant as a service, it is good for 5 years/75,000 miles.

2. Green coolant
Nissan alludes to a life of 4 years/60,000 miles

3. Mixture of Blue + Green coolants
Will result in the mixture taking the shorter life of 4 years/60,000 miles.

Take all those numbers as you will, but what is certain is that coolant needs to be changed at some point, just like engine oil. Also, I personally would go conservative and change it more frequently.

Added this to the FAQ under important threads.