a noun denoting an idea, quality, or state rather than a concrete object, e.g. truth, danger, happiness. - first google search of abstract noun
So redness is an abstract noun.
But this answer probabaly doesn't satisfy you so let me elaborate a little
I will compare colors to numbers.
You can generalize for example number 2 from looking at two apples, looking at two trees and so on.
You can also get an idea of color red by looking at red apple, red pen and so on.
But there is a difference between numbers and colors.
Numbers behave exactly the same no matter what we are doing with them and no matter what we are refering to using them (no matter if we know everything about described thing or nothing), which seems to make them outside of space-time and abstract objects.
Color seems to be an abstraction in our brains describing only certain streams of photons. And that doesn't imply them being an abstract object (in the same sense as numbers).
So there is more than one kind of abstract things.
I am oversimplifying things a little bit (but it should not matter) cuz it is only a stackexchange post.