'Is de appel groen?' (Is the apple green?) is a question, which can simply be answered by saying 'yes' or 'no'. 'De appel is groen?' (The apple is green?) is mainly a statement of surprise ('De appel is groen? Iemand anders zei net dat hij rood is.' (Someone else just now said it is red)) or, for example, when you say that the apple is red and your friend says 'The apple is green? Why can't you see that it is so very obviously green?'
To return to your question: 'De appel is groen?' is a statement of surprise, so the translation would be 'The apple is green?'. To have the translation 'Is the apple green?', the sentence should be the question 'Is de appel groen?'. Hope this was clear.