Wow, you blew my mind. You got straight to the point:
Is there any difference between consciousness of red (knowing red) and red?
No there isn't any difference between consciousness of red and red. This idea is called non-duality in Buddhism and in Advaita Vedanta philosophy.
There is no watcher watching a red perception, the watching and the "watched" perception are one in consciousness.
ordinary mind that is reified as a subject does not exist, and so phenomena are resolved within original purity. The nonduality of objects and mind is realized to be the basic space of the naturally manifest display of awakened mind. - The Precious Treasury of The Way of Abiding p.215, by Longchen Rabjam, circa 1350 A.D.
This is also why in Tibetan Buddhism consciousness is often called self-knowing awareness. in a way, the red color in consciousness knowing itself.
This is typically a hard pill to swallow for rational people, since it throws out of the window the intuition of dividing reality into objects and subjects - in this case demanding the existence of a watcher.
Rang rig or rang gi rig pa can mean literally one’s own awareness or intrinsic awareness, but in most cases in this context it indicates the distinctive feature of innate awareness, which is that it knows itself, not dualistically as a subject knowing an object, but as knowing itself by itself. This can only happen in the state beyond concepts, as conceptual mind always requires an object for cognition to occur. - The Royal Seal of Mahamudra p.301, by Kunga Tenzin, circa 1700 A.D., note by Gerardo Abboud
However in my opinion the term "knowing" used in this context is not really definable. It is used in a sense akin to watching or directly witnessing, and not the sense of the word as when one says he knows English or a particular tune, or that he knows he should be back home before midnight, and the term knowing is problematic even in its day to day ordinary use.
Isn't consciousness entirely redundant?
No, consciousness is not redundant since it is the entire "movie", not the "redundant" watcher of the movie.
There is nothing but consciousness.
Hang on to that problem, the mysterious non-duality of red and the watching of red. Keep at it again and again and again. This is the miracle all mystic traditions point to.
Does consciousness exist?
Once you realize the mysteriousness of it, you will come to see that it transcends any attempt to conceptualize it. this is why in Hindu Advaita Vedanta, and in Tibetan Buddhism it is said that consciousness cannot even be said to exist or to not exist.
Lord of the gods, you are the abode of the universe. Changeless, you are what is and what is not, and beyond the duality of existence and nonexistence. - The Bhagavad Gita, translated by Eknath Easwaran, p.199
Note that in the quote above Prince Arjuna speaks to Krishna that represents God, Brahaman, and the essence of consciousness, the soul, Atman, that are considered to be one and the same in Advaita Vedanta.
For me personally, I would tend to say that consciousness is the origin of the concept of "existence" - its "definition" by one example.
EDIT: Added a few references.