As a general guide, consider how you would say the sentence out loud. If you emphasise both the article and the other word(s), then put the article in italics:
"No, it is an orange."
If (as would be more common) you instead find that you say the article with a normal voice but put extra emphasis on the following word(s), then don't put the article in italics:
"No, it is an orange."
This general guide applies to just about any situation where you would say a particular word or words louder (see: that's an example) to indicate emphasis (there's another!). It's particularly useful in more conversational writing where you're trying to capture or replicate the non-verbal signals that spoken language includes, but as TaliesinMerlin's answer notes, don't overdo it in more formal writing.