overlay blending of two images on ipod gives a jittery result image - ios

Question

I am changing color of an image according to user( user selects the color from a list of colors), for that I am using overlay blending mode of two images to change the color. I have first image of a character(img1) and its color is to be changed, I am creating an image(resultUIImage) from the character image as shown below, and changing its pixel values to get the color which user selected. Then I overlay the img1 and resultUIImage to get my expected result(img2), but img2 is jittery; its boundary between two details of different color is a broken line. However these broken boundaries does not appear if this same project is tested for ipad. what is the possible mistake in here or is there any other way to do this ?
these images are of png format,and when I try using jpg images there is no jittering. But I cant jpg as it does not supports transparency.
-(UIImage *) getNewImageFromImage:(UIImage *)image {
const int RED = 1, GREEN = 2, BLUE = 3;
int width = image.size.width,
height = image.size.height;
uint32_t * pixels = (uint32_t *) malloc(width*height*sizeof(uint32_t));
memset(pixels, 0, width * height * sizeof(uint32_t));
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pixels, width, height, 8, width * sizeof(uint32_t), colorSpace, kCGImageAlphaPremultipliedFirst);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), [image CGImage]);
for(int y = 0; y < height; y++) {
for(int x = 0; x < width; x++) {
uint8_t * rgbaPixel = (uint8_t *) &pixels[y*width+x];
rgbaPixel[BLUE] = blueclr;
rgbaPixel[GREEN] = greenclr;
rgbaPixel[RED] = redclr;
}
}
CGImageRef newImage = CGBitmapContextCreateImage(context);
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
free(pixels);
UIImage * resultUIImage = [UIImage imageWithCGImage:newImage];
CGImageRelease(newImage);
CGSize newSize = CGSizeMake(image.size.width, image.size.height);
UIGraphicsBeginImageContext( newSize );
[ img1.image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
[ resultUIImage drawInRect:CGRectMake(0,0,newSize.width,newSize.height) blendMode:kCGBlendModeOverlay alpha:0.8];
img2.image = UIGraphicsGetImageFromCurrentImageContext();
img2.frame=img1.frame;
img2.alpha=1;
UIGraphicsEndImageContext();
return resultUIImage;
}