Assuming everything is hooked up in IB for your borderless button.
// *.h file
IBOutlet NSButton* myButton;
// *.m file
[[myButton cell] setBackgroundColor:[NSColor redColor]];
Note from the setBackgroundColor documentation:
"The background color is used only when drawing borderless buttons."
If this won't do it for you then you'll need to override NSButton and implement the drawing yourself.
Good Luck.
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