Translation:An apple is not blue: it is green or red.
I don't think it matters. You can use colons and semicolons in Esperanto.
There are various customs and rules for punctuation in Esperanto, but since details of usage differ from language to language, it's normal to see some variation when people write in Esperanto.
I don't think A or An should matter. This should be correct "Apple is not blue: It is green or red"
It may not matter in Esperanto, but it does matter in English. That sentence in English you wrote doesn't sound correct at all. You definitely need the article in the English sentence.