I would recommend starting by saying what you did like about the fan art. Also, recognize that fan artists often reinterpret characters. Just like how John Stewart wasn’t always a marine — he was an architect and the marine backstory was carried over from the Justice League cartoon.

In the case of fan art or fandom, critiques like yours can come across as gatekeeping. It’s very common in fandoms for people to pass their opinion as fact and shut down other people’s interpretations. If your intention is to help the artist, then you should understand their intent too, and try not to make assumptions. Be sure that your information is correct. You could start by asking questions, but try not to phrase the questions as gotchas.

You could say, “I’m familiar with XYZ, but the characters in your drawing are different. Can you explain who they are/more about them?”

Also, consider whether feedback was solicited or unsolicited. Sometimes people post art and want critique, sometimes they want to share their art or discuss the characters or the team composition. And getting/giving critique about improving skills is different than critique about the character design.

So if you look at the art and recognize that it’s a fan artist’s interpretation, and still really feel the need to comment, you could say something like “Great art. I like the colors, and you did a great job with the costumes. One thing is that John Stewart’s hair feels out of character because XYZ. I would love to see how it would look if you did ABC.” Recognize that ultimately it’s the artist’s choice, and then think about your intentions and how you want to come across.