An apple is red. A singular, countable noun is always preceded by an article (a, an, the).

Depends of what you mean. 'An apple is red': you are speaking about some apple you never mentioned before. Example: If an apple is red, it is likely to be sweet. 'The apple is red': you are speaking about some apple you have already mentioned. Example: There is an apple on the table. The apple is red. 'Apple is red' does not make any sense to me.

1. Apples are red. = all apples 2. The apple is red. = a. A particular apple which we know about / b. An archetypal apple is red. 3. An apple is red. = One of any of the many apples you choose is red. It seems to me that only 2a is factually correct. Many apples are green, after all.