You can't spraypaint plastic without priming it, unless you go try some of that new FUSION paint from Krylon I think.....that bonds with plastic and leaves a super glossy finish.
how well it will last under repeated button-pressings is another matter.
If they sent you the wrong color, send them back and get the right one.....ps: you realise you'll have to take the buttons apart to paint them, or you're risk gunking up the inner workings of the button right?