Nissan/nismo are generally speaking rubber nismo normaly harder rubber
nolethane etc. are urethane generally harderish than rubber
color is just a dye though urethane is available in a variety of hardness's (durometer) too, to my knowledge none of the brands advertise the durometer they use?
Not entirely skyline related but have found that things like body to chassis bushings go rubber, real chassis like a 4WD I mean, for comfort
If its a bush that holds steering angles etc urethane, especially for performance applications ie. Skylines
Just my 2c