Do you have any children that have great difficulty following routine verbal directions? Sometimes it is not because they are being stubborn, but it may be that they need extra time to auditorily process what you are asking them to do.
At times it can take months to discover what each child is trying to communicate, sometimes longer. However, there is one thing we have learned time and time again, just because a person can’t speak does not mean they do not have anything to say. We just have to take the time to learn how they are saying it.
If you are working with a child who is uninterested in writing or coloring, my advice is this: Stop worrying about coloring and start by engaging them in what they are interested in! Over time you can begin to shape their interest into a writing activity as well as build the fine motor skills they need for writing.