the.com/redness

means The quality of being red — whether it's skin flushing hot, a surface dyed scarlet, or light sitting at the long end of the visible spectrum.

from Straightforward Old English stock: 'red' plus the suffix '-ness,' which turns an adjective into an abstract noun (compare 'kindness,' 'darkness'). 'Red' itself traces back to a very old Proto-Indo-European root, 'reudh-,' that also gave rise to words for red and ruddy across many European languages — so 'redness' is less a clever coinage than a plain, functional label grammar handed us for a color humans have been naming since before writing existed.