解决方案：利用BlendMode修改图像，我们将用blending给这张图片加上另一个纯色作为tint，并保持原来的alpha通道。用Core Graphics实现：

创建一个上下文用以画新的图片

将新的tintColor设置为填充颜色

将原图片画在创建的上下文中，并用新的填充色着色（注意保持alpha通道）

从当前上下文中取得图片并返回。

最麻烦的部分可能就是保持alpha通道了。UIImage的文档中提供了使用blend绘图的方法drawInRect:blendMode:alpha:，rect和alpha都没什么问题，但是blendMode是个啥玩意儿啊…继续看文档，关于CGBlendMode的文档，里面有一大堆看不懂的枚举值，比如这样：

The blend mode constants introduced in OS X v10.5 represent the Porter-Duff blend modes.

The symbols in the equations for these blend modes are:

R is the premultiplied result

S is the source color, and includes alpha

D is the destination color, and includes alpha

Ra, Sa, and Da are the alpha components of R, S, and D

R表示结果，S表示包含alpha的原色，D表示包含alpha的目标色，Ra，Sa和Da分别是三个的alpha。

kCGBlendModeDestinationIn 模式解释：结果 = 目标色和原色透明度的加成

R = D*Sa 

AvailableiniOS2.0andlater.

DeclaredinCGContext.h.

在UIImage分类中添加以下方法：

- (UIImage*)imageChangeTintColor:(UIColor*)tintColor {

    //We want to keep alpha, set opaque to NO; Use 0.0f for scale to use the scale factor of the device’s main screen.

    UIGraphicsBeginImageContextWithOptions(self.size, NO, 0.0f);

    [tintColor setFill];

    CGRect bounds = CGRectMake(0 ,0 , self.size.width, self.size.height);

    UIRectFill(bounds);

    //Draw the tinted image in context

    [self drawInRect:bounds blendMode:kCGBlendModeLuminosity alpha:1.0f];

//kCGBlendModeDestinationIn   R = D*Sa   Available in iOS 2.0 and later. Declared in CGContext.h.

    [self drawInRect:bounds blendMode:kCGBlendModeDestinationIn alpha:1.0f];

    UIImage *tintedImage = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return tintedImage;

}