Hi @Robertjde ,
Do you want to change button's color when it is selected?
I've made a similar test for your reference:
I insert 5 buttons as an example. When you click the button, the button's color will change to red, or else it will be blue.
1)set button1's OnSelect:
Set(var,"button1")
set button1's Fill:
If(var="button1",Red,Blue)
2)set button2's OnSelect:
Set(var,"button2")
set button2's Fill:
If(var="button2",Red,Blue)
3)set button3's OnSelect:
Set(var,"button3")
set button3's Fill:
If(var="button3",Red,Blue)
4)set button4's OnSelect:
Set(var,"button4")
set button4's Fill:
If(var="button4",Red,Blue)
5)set button5's OnSelect:
Set(var,"button5")
set button5's Fill:
If(var="button5",Red,Blue)
Then the variable's value will change based on which button you click. The button's fill will change based on the variable value.

Best regards,