It seems to me that in the example given of green things that there was a time when we didn't know what made objects green. And, because we didn't know what made them green, we could conceive that green was nominal as oposed to real. But now, we do, in fact, know what green is and what makes an object green. And, therefore, we can now say that green is real as opposed to nominal.
The question of what makes both Fluffy and Kitzler cats is a bit trickier. But is the problem one of definition? Is "cat" adequately defined? Today our concept of cat is better defined. We know better today what universal cat is as a real thing than we knew at some point in the past. Is the cat real today because our concept of what a cat is more closely matches what universal cat is? Can we not examine the genetic sequence and identify the universal cat within the particular cat as a definite real universal cat?
Let's think about some really abstract notions. "One" is an concept that is well-defined. However, when we look around our world, we can define anything to be one and the thing that we previously defined to be one can become "two" by simply redefining what is "one". For example, if you take out a ruler and look at the distance known as one inch. We see that there are 12 inches on the standard ruler. But if we take the distance known as one half inch as our unit of measurement, then there are 24 half inches on the standard ruler and one inch is actually two half inches. It is unclear to us how this general notion of "one" manifests itself as real when any particular instance of "one" can, in another particular instance be "two" instead of "one". If "one" is nominal, then there exist universals that are real and there exist universals that are not real. The universal one is not like the universal green or the universal cat. It is not the case that all universals are real or that all universals are nominal.