Objective-C rendition of pixel processing found in http://stackoverflow.com/a/31661519/1271826

PixelProcessing.m

- (void)viewDidLoad {
    [super viewDidLoad];
    
    UIImage *image = [UIImage imageNamed:@"test.png"];
    self.imageView.image = [self blackAndWhiteFromImage:image];
}

- (UIImage * _Nullable)makeBlackPixelsRedInImage:(UIImage *)image {
    UInt32 black = [self pixelFromRed:0 green:0 blue:0 alpha:255];
    UInt32 red = [self pixelFromRed:255 green:0 blue:0 alpha:255];
    
    return [self processPixelsInImage:image processor:^(UInt32 pixel) {
        return pixel == black ? red : pixel;
    }];
}

- (UIImage * _Nullable)blackAndWhiteFromImage:(UIImage *)image {
    return [self processPixelsInImage:image processor:^(UInt32 pixel) {
        UInt8 red   = [self redFromPixel:pixel];
        UInt8 green = [self greenFromPixel:pixel];
        UInt8 blue  = [self blueFromPixel:pixel];
        UInt8 alpha = [self alphaFromPixel:pixel];
        UInt8 luminance = (UInt8)(0.2126 * red + 0.7152 * green + 0.0722 * blue);
        return [self pixelFromRed:luminance green:luminance blue:luminance alpha:alpha];
    }];
}

- (UIImage * _Nullable)processPixelsInImage:(UIImage *)inputImage processor:(UInt32 (^)(UInt32))processor {
    CGImageRef      inputCGImage     = inputImage.CGImage;
    CGColorSpaceRef colorSpace       = CGColorSpaceCreateDeviceRGB();
    size_t          width            = CGImageGetWidth(inputCGImage);
    size_t          height           = CGImageGetHeight(inputCGImage);
    NSInteger       bytesPerPixel    = 4;
    NSInteger       bitsPerComponent = 8;
    NSInteger       bytesPerRow      = bytesPerPixel * width;
    uint32_t        bitmapInfo       = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Little;
    
    CGContextRef    context          = CGBitmapContextCreate(nil, width, height, bitsPerComponent, bytesPerRow, colorSpace, bitmapInfo);
    
    CGColorSpaceRelease(colorSpace);

    if (context == nil) {
        return nil;
    }
    
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), inputCGImage);
    
    UInt32         *pixelBuffer      = CGBitmapContextGetData(context);
    
    for (NSInteger row = 0; row < height; row++) {
        for (NSInteger col = 0; col < width; col++) {
            pixelBuffer[row * width + col] = processor(pixelBuffer[row * width + col]);
        }
    }
    
    CGImageRef outputCGImage = CGBitmapContextCreateImage(context);
    UIImage *outputImage = [UIImage imageWithCGImage:outputCGImage scale:inputImage.scale orientation:inputImage.imageOrientation];
    CGImageRelease(outputCGImage);
    CGContextRelease(context);
    
    return outputImage;
}

- (UInt8)redFromPixel:(UInt32)pixel {
    return (UInt8)((pixel >> 24) & 255);
}

- (UInt8)greenFromPixel:(UInt32)pixel {
    return (UInt8)((pixel >> 16) & 255);
}

- (UInt8)blueFromPixel:(UInt32)pixel {
    return (UInt8)((pixel >> 8) & 255);
}

- (UInt8)alphaFromPixel:(UInt32)pixel {
    return (UInt8)(pixel & 255);
}

- (UInt32)pixelFromRed:(UInt8)red green:(UInt8)green blue:(UInt8)blue alpha:(UInt8)alpha {
    return ((UInt32)red << 24) | ((UInt32)green << 16) | ((UInt32)blue << 8) | ((UInt32)alpha << 0);
}

@end
  

@robertmryan it does take some time due to the nested loop over rows and columns or width and height. I am posting the android code here playing with Bitmap, which is working fine with only one loop, and is fast as well.

`fun replaceColor(backgroundImage: Bitmap?, foregroundImage: Bitmap?, fromColor: Int, targetColor: Int): Bitmap?
{
if (backgroundImage == null || foregroundImage == null)
{
return null
}
// Source image size
var width = backgroundImage.width
var height = backgroundImage.height

    var destWidth = foregroundImage.width
    var destHeight = foregroundImage.height

    var destPixels = IntArray(destWidth * destHeight)
    var pixels = IntArray(width * height)
    //get pixels
    backgroundImage.getPixels(pixels, 0, width, 0, 0, width, height)
    foregroundImage.getPixels(destPixels, 0, destWidth, 0, 0, destWidth, destHeight)

    for (x in pixels.indices)
    {
        destPixels[x] = if (pixels[x] == fromColor) targetColor else destPixels[x]
    }
    // create result bitmap output
    var result = Bitmap.createBitmap(destWidth, destHeight, foregroundImage.config)
    //set pixels
    result.setPixels(destPixels, 0, destWidth, 0, 0, destWidth, destHeight)
    return result
}`