There are times you are called upon to either explain or understand the various miscellany of Quartz blending modes. The simplest way to figure this out is, I find, to generate cheat sheet images to view as you read the documentation.

The following method automatically builds a set of reference images, naming them by the blend mode applied to create each one, and saving them to the application Documents folder.

- (void) generateBlendingImages
{
    NSArray *names = @[@"kCGBlendModeNormal", @"kCGBlendModeMultiply", @"kCGBlendModeScreen", @"kCGBlendModeOverlay", @"kCGBlendModeDarken", @"kCGBlendModeLighten", @"kCGBlendModeColorDodge", @"kCGBlendModeColorBurn", @"kCGBlendModeSoftLight", @"kCGBlendModeHardLight", @"kCGBlendModeDifference", @"kCGBlendModeExclusion", @"kCGBlendModeHue", @"kCGBlendModeSaturation", @"kCGBlendModeColor", @"kCGBlendModeLuminosity", @"kCGBlendModeClear", @"kCGBlendModeCopy", @"kCGBlendModeSourceIn", @"kCGBlendModeSourceOut", @"kCGBlendModeSourceAtop", @"kCGBlendModeDestinationOver", @"kCGBlendModeDestinationIn", @"kCGBlendModeDestinationOut", @"kCGBlendModeDestinationAtop", @"kCGBlendModeXOR", @"kCGBlendModePlusDarker", @"kCGBlendModePlusLighter"];

    CGRect rect = CGRectMake(0, 0, 100, 100);
    UIBezierPath *shape1 = [UIBezierPath bezierPathWithOvalInRect:rect];
    rect.origin.x += 50;
    UIBezierPath *shape2 = [UIBezierPath bezierPathWithOvalInRect:rect];

    for (int i = kCGBlendModeNormal; i <= kCGBlendModePlusLighter; i++)
    {
        NSString *name = [names[i - kCGBlendModeNormal] stringByAppendingPathExtension:@"png"];
        UIGraphicsBeginImageContext(CGSizeMake(150, 100));

        [greenColor set];
        [shape1 fill];

        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), i);

        [purpleColor set];
        [shape2 fill];

        UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
        NSData *data = UIImagePNGRepresentation(image);
        [data writeToFile:DOCS_PATH(name) atomically:YES];
        UIGraphicsEndImageContext();

    }
}