Set up each button to perform a script. If it's currently performing a single step selected in button setup, put that step in to a script and use perform script.
For each button's script, include steps that change the value of the same global variable:
Set variable [$$ButtonClicked ; value: "Button 1" ]
Then test this variable in a conditional format expression that changes the fill color of the button:
$$ButtonClickec = "button 1"
might be the expression that changes the fill color when button 1 is clicked.