Buying coloring for children, look for task-coloring, in which you need something painted in a specific color, something to finish, something to paste, draw a line through the maze, etc.
Images for older children may already contain small parts, complex narrative compositions with greater color palette. The outline is more subtle. These can be supplemented with coloring tasks. For example, to write the missing word, or paste a sticker, finish drawing, connect the dots.