Find the lightest (or darkest) color in an image by sampling pixels along both diagonals.

Color Sampling Experiment

CGImageRef rawImageRef = [image CGImage];

CFDataRef imageDataRef = CGDataProviderCopyData(CGImageGetDataProvider(rawImageRef));
const UInt8 *rawPixelData = CFDataGetBytePtr(imageDataRef);

NSUInteger imageHeight = CGImageGetHeight(rawImageRef);
NSUInteger imageWidth = CGImageGetWidth(rawImageRef);

int brightness = 0;
int red = 0;
int green = 0;
int blue = 0;

int x = 0;
int y = 0;
int p = 0;
int r;
int g;
int b;
int br;

float ratio = (float)imageHeight / (float)imageWidth;

for (int i = x; i < imageWidth; i++) {
    x = i;
    y = floorf(x*ratio);
    p = ((imageWidth * y) + x) * 4;

    r = (unsigned int)rawPixelData[p];
    g = (unsigned int)rawPixelData[p + 1];
    b = (unsigned int)rawPixelData[p + 2];
    br = (int)sqrt(r * r * .241 +
                       g * g * .691 +
                       b * b * .068);

    if (br > brightness) {
        brightness = br;
        red = r;
        green = g;
        blue = b;
    }

    // Check the other diagonal
    y = imageHeight - y;
    p = ((imageWidth * y) + x) * 4;

    r = (unsigned int)rawPixelData[p];
    g = (unsigned int)rawPixelData[p + 1];
    b = (unsigned int)rawPixelData[p + 2];
    br = (int)sqrt(r * r * .241 +
                   g * g * .691 +
                   b * b * .068);

    if (br > brightness) {
        brightness = br;
        red = r;
        green = g;
        blue = b;
    }

    if (brightness == 0)
        break;
}

CFRelease(imageDataRef);

UIColor *color = [UIColor colorWithRed:red/255.0 green:green/255.0 blue:blue/255.0 alpha:1];