Determine whether the given UIImage is dark or light in a standard way.
If you're only doing black or white text, use the color brightness calculation above. If it is below 125, use white text. If it is 125 or above, use black text.
edit 1: bias towards black text. :)
edit 2: The formula to use is ((Red value * 299) + (Green value * 587) + (Blue value * 114)) / 1000.
参考资料： http://stackoverflow.com/questions/2509443/check-if-uicolor-is-dark-or-bright https://gist.github.com/justinHowlett/4611988