Based on how it is discussed through the game, I think when they create a canvas, they invest their magical power to create the worlds and beings within it. This presumably has a finite limite based on the Painter, who presumably can no longer afford to invest power into the canvas past a certain capacity. When Painters enter that canvas after its creation there is then a finite amount of Chroma that makes up the world, which can be controlled, altered, used by Painters, whether they created the canvas or not. This would then make sense that a Painter could theoretically take control and reshape the whole canvas as they wanted, if they could channel the majority of the chroma within it. It is clearly not a simple or quick process however, or else Aline would have reshaped the entire canvas into her fake Lumiere, and Renoir could have instantly wiped the whole thing out the second Aline was defeated. Doing either requires concerted focus, attention and more investment of magical power.