A reference is a target
Printing the coloured version alongside the outline turns colouring into a matching task. That is genuinely useful when you are teaching the colour of something — a fire engine, a lemon, a zebra — or when a child needs the security of a right answer.
It also helps with colour vocabulary: the child has something to point at while naming it.
When to switch it off
If the goal is invention, a reference actively gets in the way. A purple elephant is a good outcome in an open-ended session and a wrong answer in a matching one, and the sheet decides which session it is.
For free-choice colouring, print the outline alone. The child has more to bring to the page when nothing on it is already decided.
A practical middle ground
Project the reference on the board rather than printing it. Children who want the guidance look up; children who do not, do not. One decision covers a mixed group.