A big apple / An big apple / Big an apple?

As Always, your comments are highly regarded. That being said, I reviewed what I wrote with your reply about "a red apple". I think "red" or "green" would be an unfair assumption. We were simply talking about one apple with no mention of color. An apple is not the same as a red apple. I did say, "an" is required if the following noun begins with a vowel. But red - is an adjective. An- apple is a noun which begins with a vowel. Just so it is noted and clarified. Thanks