Trump was surrounded by children coloring flags at a pediatrics hospital when the photo was taken. While Trump’s appeared to be incorrect, other children present successfully completed the task.

It is of course possible and far preferable that Trump did so intentionally, for some reason.

Newsweek – Donald Trump colored the American flag in wrong
Rawstory – Trump managed to botch a photo-op with Ohio kids by coloring his American flag wrong
Snopes – Did President Trump Incorrectly Color the American Flag?