ir

It's not strictly "correct," but I LOVE IT. KEEP IT. OR I WILL BE UPSET. :)

It is perfectly clear and easy to understand. It does not strike me as illiterate or ignorant at all. It is reminiscent of the unrelated word "irredeemable."

Actually I am not sure what the correct word would be! You can "redden" something (and then it "has been reddened"). I think most native English speakers would tend to use the "un-" prefix because it is usually seen as the default. So "unreddenable."

But let me say this again... your title is very artistic in itself. I think this is just one of many occasions when what is "correct" in one sense must give way to what is correct and pleasing artistically.

"ir-" only ever happens before "r", but that doesn't mean all words starting with "r" must be negated using "ir-".

Examples negated with "un-" include unrecognizable, unrecommendable, unrecordable, unreliable, unremarkable, unrentable, unrepealable, unresolvable, unsaleable, and unsatisfiable.

mpt, you must tell us what you think the correct English word would be to describe something that cannot be reddened!

Can it be that such a concept requires more than one word to be correctly articulated in English?

Oh, AC, that's passe... the "one word" necesary has now been coined. By Goossun. :)