The Opacity setting is not for the button itself but for the highlight that the button uses.
Add a button rectangle to a blank menu, and add some text to it. Use the property inspector to make sure the button text is included in the highlight, and then (just above the opacity slider) select a colour from the drop down. Your text will appear in the colour you choose. Be careful though - this is not the text, but the highlight of the text. The opacity slider now will let you fade the highlight colour and show you the original text underneath.
If you are using the default settings, your text will be white and the highlight colour (selected highlight setting) will also be white. Change that to red and move the opacity slider - the text appears gradually more pink until it is only white showing.
Now add a second button and repeat the same exercise. See how one button text looks red and the other looks white? That's because one button is showing as 'selected'. Now move the opacity slider until the selected button looks like it has pink text - then click on the other button... it will also show as pink.
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lafcpug mod