The key is to understand whether he can color carefully, but will not, or whether he cannot do it at all. Those are two very different situations.
Assuming he can, my advice is, let it go. As an elementary school student, this was ground I refused to yield, and I was quite contented to get U's or C's/D's in penmanship, so long as all my other marks were good. If the teacher could read the words, that was all that mattered, and I couldn't be bothered to make my work look pretty. My hand moved too slowly for my brain as it was. Later on, that turned out to be the right habit, as working quickly and barely legibly allowed me to maintain a mental flow state while writing essays, which meant I was able to churn out more and higher-quality content during timed/untimed writing exercises.
DD9 is skipped into 5th grade and in a G/T daily pull-out for ELA. Among her homework assignments last week was an instruction to draw a picture, and integrate her spelling words into it. We shared a mutual eye roll, and I gave her leave to put in minimal effort, which she did. She made a random loopy line, then wrote her words in places along the line.
DD had always had beautiful writing until she reached K, so we knew that for her it was never a question of "cannot," and her penmanship suffered as a clear symptom of her discomfort with school. In order to make sure she played the game and showed what she was capable of, I did point out errors when she rushed through math homework and made obvious mistakes, and I did make her rewrite anything that was illegible. But I never pushed her to return to her previous levels of penmanship, and always accepted anything I could read as "good enough." She's starting to self-motivate back in that direction this year.
