为何Android的点9图四周的颜色只能是白、黑或透明

http://androidren.com/index.php?qa=363&qa_1=%E4%B8%BA%E4%BB%80%E4%B9%88android%E7%9A%84%E7%82%B99%E5%9B%BE%E5%9B%9B%E5%91%A8%E7%9A%84%E9%A2%9C%E8%89%B2%E5%8F%AA%E8%83%BD%E6%98%AF%E7%99%BD-%E9%BB%91%E6

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