Hi @jNelly,
Do you want to make the icon of the selected item change colour when you click a button outside the Gallery?
If you could accept adding a column like Status to make difference, that's would be easier to achieve your need.
1). Add a Text column to your list
2). Once you select an item from the Gallery, click the button to update the corresponding status as Color. Set the Onselect property of the button outside the gallery as below:
Patch(ListName,Gallery9.Selected,{Status:"Color"})
3). Make the Fill property of the icon as below:
If(ThisItem.Status="Color",Red)