UIImage method that makes color histogram from the image and sort them by number of occurences. Clone and edit for your specific purpose.

UIImage+MostUsedColor.m

- (NSMutableData *)mutableRGBAData {
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    CGSize scaledSize = (CGSize){
        .width = self.size.width * self.scale,
        .height = self.size.height * self.scale,
    };

    NSUInteger const bytesPerPixel = 4;
    NSUInteger const bitsPerComponent = 8;
    NSUInteger const bytesPerRow = bytesPerPixel * scaledSize.width;
    NSMutableData *bitmap = [[NSMutableData alloc] initWithLength:scaledSize.height * scaledSize.width * bytesPerPixel];
    
    CGContextRef context = CGBitmapContextCreate(bitmap.mutableBytes,
                                                 scaledSize.width, scaledSize.height,
                                                 bitsPerComponent, bytesPerRow,
                                                 colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    CGRect rect = (CGRect) {
        .origin = CGPointZero,
        .size = scaledSize,
    };
    CGContextDrawImage(context, rect, self.CGImage);
    
    CGColorSpaceRelease(colorSpace), colorSpace = NULL;
    CGContextRelease(context), context = NULL;
    
    return bitmap;
}


- (void)loadMostUsedColors {
    NSData *bitmap = [self mutableRGBAData];
    const unsigned char *bytes = bitmap.bytes;
    NSCountedSet *colorHistogram = [[NSCountedSet alloc] initWithCapacity:32];
    
    for (NSUInteger index = 0; index < bitmap.length; index += 4) {
        UIColor *pixelColor = [UIColor colorWithRed:bytes[index + 0] / 255.
                                              green:bytes[index + 1] / 255.
                                               blue:bytes[index + 2] / 255.
                                              alpha:1];
        [colorHistogram addObject:pixelColor];
    }
    
    CGSize scaledSize = (CGSize){
        .width = self.size.width * self.scale,
        .height = self.size.height * self.scale,
    };

    NSUInteger minimalOccurence = (scaledSize.width * scaledSize.height) / 100; // 1 %
    NSMutableDictionary *colorToOccurence = [[NSMutableDictionary alloc] initWithCapacity:colorHistogram.count];
    for (UIColor *color in colorHistogram) {
        NSUInteger occurence = [colorHistogram countForObject:color];
        if (occurence >= minimalOccurence) {
            [colorToOccurence setObject:@(occurence) forKey:color];
        }
    }
    
    NSArray *mostUsedColors = [[[colorToOccurence keysSortedByValueUsingSelector:@selector(compare:)] reverseObjectEnumerator] allObjects];
    
    // `mostUsedColors` is an array of colors with more than 1% share of the image
    // `colorToOccurence` is a dictionary with UIColor keys and their number of occurences as values
}

To incorporate some tolerance or average, you may want to add some rounding to the raw values and/or use different storage type, than plain counted set.

Note: Transparent pixel in image will result in black color. Edit the code for your needs.