UIColor
+ (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;