Let’s put an end to this. Nhu provided the answer: coeruleus is a misspelling, but not a blatant orthographic error. Therefore it should be maintained. We know that in Ancient Latin, the two diphthongs ae and oe had very different pronunciations and would never have been confused. But by classical times the sounds had become very similar. In Vulgar Latin of the time, especially in an unstressed syllable, there was probably no longer any difference. Thus different ‘phonetic’ spellings could coexist. Probably none but the most erudite maintained the distinction. The confusion of the ligatures couldn’t have occurred until minuscules were used, several centuries later. Rather, the confusion between oe and ae must have been a product of Early Late Latin, where they both had the same pronunciation. Caerulea is feminine: Primula marginata caerulea vs. Calochortus caeruleus