[UIColor colorWithRed: green: blue: alpha:] 颜色值范围都是在0.0~1.0之间的，并不是我们误认为的0~255。

正确用法：

[UIColor colorWithRed:240.0/255 green:240.0/255 blue:240.0/255 alpha:1.0];

colorWithRed:green:blue:alpha:
Creates and returns a color object using the specified opacity and RGB component values.

Declaration
OBJECTIVE-C

• (UIColor *)colorWithRed:(CGFloat)red
  green:(CGFloat)green
  blue:(CGFloat)blue
  alpha:(CGFloat)alpha

Parameters

red

The red component of the color object, specified as a value from 0.0 to 1.0.

green

The green component of the color object, specified as a value from 0.0 to 1.0.

blue

The blue component of the color object, specified as a value from 0.0 to 1.0.

alpha

The opacity value of the color object, specified as a value from 0.0 to 1.0.

Return Value

The color object. The color information represented by this object is in the device RGB colorspace.

Discussion
Values below 0.0 are interpreted as 0.0, and values above 1.0 are interpreted as 1.0.

Import Statement
Availability
Available in iOS 2.0 and later.