+ (UIColor *)colorWithWhite:(CGFloat)white alpha:(CGFloat)alpha;
white （灰度）
The grayscale value of the color object, specified as a value from 0.0 to 1.0.
blackColor（黑色）：0.0 white
darkGrayColor（深灰）：0.333 white
grayColor（灰）：0.5 white
lightGrayColor（浅灰）：0.667 white
whiteColor（白色）：1.0 white
alpha （透明度）
The opacity value of the color object, specified as a value from 0.0 to 1.0.
0.0：透明
1.0：不透明
+ (UIColor *)clearColor; // 0.0 white, 0.0 alpha
Returns an NSColor object whose grayscale and alpha values are both 0.0.
灰度和透明度都为0
+ (UIColor *)colorWithRed:(CGFloat)red green:(CGFloat)green blue:(CGFloat)blue alpha:(CGFloat)alpha;