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

//  UIImage+Tint.m

#import "UIImage+Tint.h"

@implementation UIImage (Tint)
- (UIImage *) imageWithTintColor:(UIColor *)tintColor
{
    return [self imageWithTintColor:tintColor blendMode:kCGBlendModeDestinationIn];
}

- (UIImage *) imageWithGradientTintColor:(UIColor *)tintColor
{
    return [self imageWithTintColor:tintColor blendMode:kCGBlendModeOverlay];
}

- (UIImage *) imageWithTintColor:(UIColor *)tintColor blendMode:(CGBlendMode)blendMode
{
    //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:blendMode alpha:1.0f];

    if (blendMode != kCGBlendModeDestinationIn) {
        [self drawInRect:bounds blendMode:kCGBlendModeDestinationIn alpha:1.0f];
    }

    UIImage *tintedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return tintedImage;
}

@end