I recognise it because I have had the subjective experience of 'redness'. So whether it exists for any other human I cannot say but I am certainly 100% certain it exists for me. However I should add that I can't fully define what this experience is. Though people say the same of love!

I'll appeal to authority in that scientists and philosophers today in all of the worlds universities and those in the past have determined to understand this phenomenon. That it exists is a given, what it is, is more murky. Again it's not me saying this.