The golden-haired teen is said to be keen on trying out a black barnet but has been told in no uncertain terms that it's a no-go.
A source told The Sun: "Simon still controls a lot about the band down to the finest detail.
"Niall was desperate to get rid of his blond locks and go black just for an experiment, like loads of normal teens... He put in a text to [Simon] asking if he could change his hair colour.
"Simon was straight back on to him with a short reply, 'No way'."