We still need a little more information. Can only 1 label be green at all times, or can the user select multiple entries?
If only 1 label needs to be green, we can achieve that in the following way. Use a rectangle in the gallery that has the desired green color. Then go to the visibile property and set its value to the following formula
If(ThisItem.IsSelected,true,false)
What this does is that whenever you select an entry, ThisItem.IsSelected will evaluate to true for that specific entry and, thus, result in 'true' and the rectangle becomes visible.
Let me know if this solves your question.