Student or Learner
In a sentence on fiction, if we say something that we know definitely doesn't exist in reality, should we use past tense or present tense? Here is an example:
Is there any filter that can keep my mind clean?
We know that there is no such a filter that can do this, so should I use Is or Was in the sentence above?
This type of jocular response, meaning 'yes - obviously' is not fiction. A well known version of this is 'Is the Pope (a) Catholic?' 'Fictional' versions, for example, 'Is the sky green?' or 'Is the Pope an atheist?' are not used.
Even so, I'm saying that I still don't understand why the question should be: "Was the sky blue?", rather than "Is the sky blue?" That is, where did the idea to ask whether the question should be in the past tense come from?
For the OP, if you ever want to ask whether, hypothetically, the sky is, or was, green, you'd use 'is' or 'was' with their usual usages. That is, it would depend on whether you wanted to know whether the sky is/was hypothetically green now or in the past.
You might be confusing this with the use of the subjunctive in contrary to fact situations; and assuming that you use the past tense form "was" in such a case. But this is confused for several reasons.