I think it probably means that he is just bored with coloring books and wants to produce his own art. It is possible that he has heard a teacher or someone malign coloring books and thinks that by ignoring the pictures he is doing what he should. To some people, coloring books are an anathema. I am not in that group, as I feel that they help develop eye-hand coordination and fine motor control. However, if they provide the only opportunity for drawing and coloring that a child has, I think they are more harmful than helpful.
Try giving him some finger-paints and brush paints and sheets of blank paper. I save sheets of paper that come out of my printer with maybe only one or two lines on them for my granddaughters to use for drawing and coloring. If you have an easel for him to use, that is fine. However, putting the paper down on the kitchen floor on top of newspapers will work just about as well. Or, if he says he wants to color not paint, give him blank paper and crayons or pencils.
There is certainly nothing to worry about in the reduced interest he is showing in coloring. It could be either a phase, as you suggest, or it could simply be an indication that this is not where his interests lie.
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